Subscribe to a Weekly Series

By Rabbi Yitzchak Etshalom | Series: | Level:

5. If he was engaged in Talmud Torah and the time for K’riat Sh’ma arrived, he interrupts [his study] and reads [K’riat Sh’ma] and recites the B’rakhot beforehand and afterwards.

If he was involved in *Tzor’khei Rabim* (lit. “community needs”), he should not interrupt; rather, he should complete their affairs and then read – if there is time left in which to read.


6. I he was involved in eating, or he was in the public bath, or he was involved in getting a haircut, or if he was examining hides (in a tannery) or if they were engaged in a court case, he completes [the activity] and afterwards reads K’riat Sh’ma. If he was concerned that the time for K’riat Sh’ma might pass and he interrupted [the activity] and read, this is praiseworthy.

[RABD: “If he was involved in eating…” The RABD wrote: No! Rather he must interrupt [the activity] and read, even if there is enough time to read [afterwards], because [K’riat Sh’ma] is d’Orayta (mandated by the Torah); and if there isn’t enough time remaining, he [even stops] for Tefillah, which is d’Rabanan (mandated by the Rabbis). This idea is found in Tractate Sukkah…and is hinted to in Tractate Berakhot.]

The Exemption of “Osek beMitzvah”

Yitzchak Etshalom



The Mishna in Sukkah (2:7) states: *Shluhei Mitzvah P’turim Min haSukkah* – emissaries, on their way to perform a Mitzvah, are exempt from the Mitzvah of living in a Sukkah. The Gemara (Sukkah 26a) quotes a Baraita which explains that those who are travelling in order to fulfill a Mitzvah, even if they are only travelling during the days of Sukkot, are exempt from the Mitzvah even during the nights of Sukkot.

The Gemara in Sukka also quotes the following Halakha in the name of R. Hananya b. Akavia:

“Those who are writing S’farim (= Sifrei Torah), Tefillin and Mezuzot, they, their salesmen and their salesmens’ salesmen (i.e. all the middlemen from the scribe to the consumer) and all who are involved in *M’lekhet Shamayim* (lit. “God’s work) – including those who sell Tzitzit, are exempt from K’riat Sh’ma, Tefilla, Tefillin and from all Mitzvot in the Torah – to support the words of R. Yossi haG’lili, since R. Yossi haG’lili said: *Ha’Osek b’Mitzvah Patur min haMitzvah* – (lit. – “one who is engaged in a Mitzvah is exempt from another Mitzvah.”).”

The Mishna in Berakhot (2:4) states that a bridegroom is exempt from K’riat Sh’ma for the first four nights – or until he consummates the marriage, whichever comes first – because he is “tarud bemitzvah” – he is thinking about and concerned regarding the Mitzvah he is about to perform. It is instructive to note that the Mishna only discusses the grooms’s exemption with regards to K’riat Sh’ma; yet, the Gemara (Sukka 25a) ties this Halakha in with the general rule of Ha’Osek b’Mitzvah…. This may mean that the rule of Ha’Osek b’Mitzvah… is limited regarding from which Mitzvot we are exempted while engaged in others.

From these three Halakhot, we learn that both people who are actively engaged in the performance of a Mitzvah (or, at least, certain Mitzvot) and also those who are on their way to perform a Mitzvah are exempt from doing other Mitzvot (or certain Mitzvot).



The Gemara (Sukka 25a) presents two sources for the Halakha of Ha’Osek b’Mitzvah…

(a) “(You shall teach these words to your children and speak them) *B’Shiv’t’kha b’Veitekha uv’Lekht’kha vaDerekh* – when you are doing your own sitting at home and when you are doing your own walking on the way” – From *B’Shiv’t’kha b’Veitekha*, we exclude/exempt someone who is *Osek b’Mitzvah* – and from *uv’Lekht’kha vaDerekh* we exclude/exempt a bridegroom. The Gemara explains that because of the phrasing *shiv’t’KHA* – your own sitting, we infer that your are only obligated to do a Mitzva when you are otherwise unoccupied with Mitzvot – when you are involved with your own affairs. As Tosafot (s.v. uv’Lekh’t’kha) points out, the first verse teaches us that someone who is presently travelling to perform a Mitzvah is exempt; whereas the bridegroom is just sitting at home, worrying and concerned about his own upcoming consummation – therefore we need two phrases, to teach us that even someone who is “engrossed and concerned” with an upcoming Mitzvah is exempt from other Mitzvot.

(b) In Bamidbar (Numbers) 9, we read about some people who were impure and unable to perform the Pesach sacrifice. The Gemara presents several opinions about who they were and to whose burial they attended – which caused their impurity. One of the options is that they were involved with a burial, such that the seventh day of impurity came out on Erev Pesach – and, even though this prevented them from participating in the Pesach, they did not desist from burying the person a week earlier. The application is that their earlier involvement with burial was proper, even though it excluded them from a later Mitzvah.

Significantly, neither of these sources relates to someone who is presently performing a Mitzvah – that he is exempt from another one. Both relate to time-delay exemptions; either exempting you from Mitzvot while you are engrossed in the details of an upcoming Mitzvah, or allowing/obligating you to perform a specific Mitzvah, even though its performance will exclude you from a later Mitzvah.

The first source is a bit difficult – how do we infer from a verse which relates uniquely to K’riat Sh’ma (“when you sit in your house…”) that involvement in any Mitzvah exempts you from doing another? We will file this question and respond to it later on in the shiur.



In investigating the rule of Ha’Osek b’Mitzvah…, there are five issues which demand clarification:

(a) which Mitzvot “exempt” us from performing others? In other words, does involvement in any Mitzvah exempt you, or only specific Mitzvot?

(b) For which Mitzvot do we not interrupt? In other words, when engaged in a Mitzvah, do we never interrupt for another Mitzva, or do we interrupt for some Mitzvot, and not for others? (this is the inverse side of the first question)

(c) How parallel are the laws of interrupting Mitzvah-performance and Mitzvah-preparation? In other words, do the same parameters apply to interrupting performance of a given Mitzvah and interrupting travel/preparation for a given Mitzvah?

(d) Why is Talmud Torah treated differently (both here and elsewhere – see MT Talmud Torah 3:4) than other Mitzvot, such that we do interrupt Torah study to perform other Mitzvot?

(e) What is the rationale for the rule of Ha’Osek b’Mitzvah Patur min haMitzvah?



Before directly addressing these questions, there are two Rambams (selections from MT) which seem to stand in contradiction – resolution of which may be a key to unlocking the basis of the rule: Ha’Osek b’Mitzvah Patur min haMitzvah.

In Hilkhot Talmud Torah 3:4, Rambam rules that if he had the option of either performing a Mitzvah or Talmud Torah; if the Mitzvah could be done by someone else, he should not interrupt his study. If not, he should perform the Mitzvah and return to his study. (See our discussion at this Halakha). This indicates that Talmud Torah takes a “back seat” to other Mitzvot, at least from a perspective of *d’chiyah* – one being pushed away in favor of the other. (From the beginning of Rambam’s phrasing, he seems to be addressing a situation pre facto – before the person has begun his study; however, from the rest of the Halakha, it is clear that this rule applies even after the person has begun his study. Even if he is in the middle of study – and certainly if he has not yet begun – he interrupts this study in order to perform the Mitzvah which could not be done by anyone else.) Even further – Talmud Torah takes this “back seat” to ALL other Mitzvot; since, if we are involved in any other Mitzvah, we do not interrupt it for another Mitzvah. For instance, if someone is involved in taking care of a found item, which he is caring for until the owner claims it, he is exempt from giving Tzedaka (BT Bava Kamma 51b); on the other hand, if he is studying and someone in need approaches him, he is obligated to interrupt his study in order to give Tzedaka.

Parenthetically, there are two types of situations where a Mitzvah would need to be done by this person alone; if there is no one else around (or no one capable of doing it); or if it is a *Mitzvah sheb’Gufo* – a Mitzvah which must be done by that individual. For instance, Tzedaka need not be done by everyone; if the needy person is cared for by another, the end of that Mitzvah has been met – the man has food in his stomach, a roof over his head etc. However, if the time for eating Matzah or shaking a Lulav comes, these are not Mitzvot which are satisfied by the result of them BEING DONE; they must be done by the self-same person who is commanded. I must shake Lulav and eat Matza – so I must necessarily interrupt my study for these Mitzvot.

In Hilkhot Ishut (15:2), Rambam rules that a man is obligated in the Mitzva of having children – “be fruitful and multiply” – from the age of 17. By the age of 20, he is considered to have “ignored” this Mitzva if he has not yet married (and, presumably, begun his attempts to sire children.) “…However, if he was engaged in Torah study *v’Tarud Bah* – (and engrossed in it), and was afraid to marry, lest he become too busy with working to support [his wife] – which will take him away from Torah – he is allowed to delay [marriage]. Because Ha’Osek b’Mitzvah Patur min haMitzvah – all the more so regarding Talmud Torah.”

Here, Rambam makes an about-face, placing Talmud Torah above all other Mitzvot, insofar as it should most certainly not be interrupted in order to perform another Mitzvah.



Talmud Torah is, as Rambam intimates, greater in value than any other Mitzvah. The question is not so much why we allow someone to delay marriage and children (and, in some cases, put it off altogether, see MT Ishut 15:3) – the question is why Talmud Torah is so easily interrupted for other Mitzvot.

Immediately before presenting the Halakha that Talmud Torah is interrupted for all other Mitzvot, Rambam states:

No other Mitzva among the Mitzvot can be equated to Talmud Torah; rather Talmud Torah is equal to all of the Mitzvot combined; for study leads to action. Therefore, study takes precedence over action in all cases.

Here, Rambam is establishing the place of Talmud Torah within the larger matrix of Mitzvot: Talmud Torah is not another one of the 613 Mitzvot – it cannot be reckoned in a list with Lulav, Shofar, returning lost items etc. Although it is one of the 613, it plays a different role than any other, in that all other Mitzvot depend upon Talmud Torah for their fulfillment. How can we properly observe Shabbat, avoid Hametz, conduct ourselves properly in business etc. without learning about those specific areas of Jewish law? Therefore, Talmud Torah is “balanced” equally with all other 613 Mitzvot. Even the Mitzvah of Talmud Torah needs to be researched, analyzed, discussed and taught (as we did in this forum). Talmud Torah is certainly the “greatest” Mitzvah of all – yet, it’s relationship with other Mitzvot may be misleading. Since the goal – or one of the goals – of learning is application and action, and regarding one who studies but does not fulfill the mandates of his learning, the Rabbis stated that “it would have been better if he had been stillborn” (JT Shabbat 1:2), it seems clear that it would be violating the goal of Talmud Torah to ignore a Mitzvah which presents itself to the student. Not only do we first perform the Mitzvah and then begin study; we even interrupt study in order to perform the Mitzvah – this is not due to the “weakness” of Talmud Torah relative to other Mitzvot, rather on account of its greatness – Talmud Torah implies action and, the greater the level of study, the greater the commitment to realizing the implications of that study should be.

This is all predicated upon the assumption, as Rambam writes, that after performing the Mitzvah which interrupted him from study, he “returns to his study.” Here is where the unique situation addressed in Hilkhot Ishut comes into play. When we speak about a Mitzvah which takes a defined amount of time and energy to complete (e.g. giving Tzedaka, shaking Lulav), it is reasonable to say that we should interrupt study in order to fulfill that – as that is the great lesson of learning – and then we return to the “source” of that commitment and inspiration. However, when the Mitzvah involvement will engross us permanently and intensely in something new – then we must take a different perspective. Here, the great value of Talmud Torah places it “above” other Mitzvot. Since the rule is Ha’Osek b’Mitzvah Patur min haMitzvah, if one is engrossed in learning, he is exempt from involvement in another Mitzva which will take him away from that intense learning on a constant basis.



Rabbenu Meshulam (commentary on Sukkah 25a) points out that the phrase of the rule is Ha’Osek – meaning, one who is engrossed and involved. In other words, just doing a Mitzva is not sufficient reason for an exemption – for example, it is quite easy to give Tzedaka while wearing Tzitzit without it taking away one iota from the Mitzvah of Tzitzit (Ritba, Sukka 25a s.v. Shluhei). It is the *Osek*-nature of the involvement, that we are concerned that if you turn away from the Mitzva in which you are engrossed and which takes focus – in order to do another – that that may impair your ability to perform the first Mitzvah completely.

Rashi (Sukka 26a s.v. Hol’khim) makes this point, when he explains that the reason that those who are travelling during the day are exempt at night from Sukkah, because “even though they are only travelling during the day; since they are concerned and worried about the Mitzvah and its appurtances, they are exempt from the Mitzvah [of Sukkah].” In other words, since by concerning themselves with constructing or locating a Sukkah during the evening (while they are not directly involved in whatever Mitzvah they are travelling to perform), they will likely lose their focus on the next day’s travels and activities – they are exempt even at night from Sukkah.

This may work in the inverse – it may be that we may interrupt being Osek b’Mitzvah for certain other Mitzvot which do not, in any way, take away from the concentration necessary for the first one. The Gemara (Sukkah 25b-26a) states that a groom, his attendants and all of the wedding party are exempt (for the first seven days) from Tefilla and Tefillin, but are obligated to say K’riat Sh’ma. Although Rashi claims that the author of this statement does not accept the rule of Ha’Osek b’Mitzvah (see Rif, HaMaor and RABD here), Ritba does suggest that this is the reason. Therefore, he says, since it is possible to interrupt the celebration and focus long enough to recite Sh’ma properly (with total concentration only needed during the recitation of the first verse) – we do obligate them to interrupt their Mitzvah of celebration for this; however, since both saying Tefillah and wearing Tefillin require much more concentration, we exempt them from these, due to their involvement in the Mitzvah of celebration.

In summary, it seems that the central issue of Ha’Osek b’Mitzvah is one of focus; if someone is involved in preparations or concerns about an upcoming Mitzvah, that exempts him (even disallows him – see Ritba in Sukkah) from “leaving” that level of focus in order to perform another – unless that performance would in no way lessen the fulfillment of the first Mitzvah. The rule is not stating that while you are performing one Mitzvah, you need not interrupt it to perform another. As the Or Zarua’ (#299) points out, there is no need for a verse or special Halakha to teach that; for “what reason would we have to think that he would leave one Mitzvah in which he is involved, in order to become involved in another Mitzvah – even a “lighter” Mitzvah would not be interrupted for a “serious” Mitzvah, since no one knows the reward for Mitzvot (i.e. which “weighs” more).


We can now look at the five issues raised earlier and attempt to answer them:

(a) which Mitzvot “exempt” us from performing others? In other words, does involvement in any Mitzvah exempt you, or only specific Mitzvot?

A: Only those Mitzvot which take some type of focus and concentration.

(b) For which Mitzvot do we not interrupt? In other words, when engaged in a Mitzvah, do we never interrupt for another Mitzva, or do we interrupt for some Mitzvot, and not for others? (this is the inverse side of the first question)

A: Same as before – we will not interrupt to perform a Mitzvah, unless it can be done without taking away from the concentration needed to complete the first one. For example, although travelers on a mission for a Mitzvah (e.g. to greet their teacher, to ransom hostages) are exempt from Sukkah even during the time of day when they are not traveling, nevertheless, they are still obligated to say K’riat Sh’ma – since that can be accomplished without taking away from their journey and its preparations and concerns.

(c) How parallel are the laws of interrupting Mitzvah-performance and Mitzvah-preparation? In other words, do the same parameters apply to interrupting performance of a given Mitzvah and interrupting travel/preparation for a given Mitzvah?

A: From our sugyot, it seems that there is a total parallel – such that it doesn’t matter if you are doing a Mitzvah or are involved in one (preparing, traveling etc.) or even engrossed in thinking about one (e.g. the groom – although no other example comes to mind); in any case, you are exempt from other Mitzvot, under the qualifications outlined above.

(d) Why is Talmud Torah treated differently (both here and elsewhere – see MT Talmud Torah 3:4) than other Mitzvot, such that we do interrupt Torah study to perform other Mitzvot?

Because the goal of Talmud Torah is action…but see the next section.

(e) What is the rationale for the rule of Ha’Osek b’Mitzvah Patur min haMitzvah?

See next section.



The one question we have left is – how do we get from a verse which exempts someone from saying K’riat Sh’ma if they are involved in other “Heavenly” activities – to Ha’Osek b’Mitzvah Patur min haMitzvah?

I believe that there is a logic flow here:

[Keep in mind that the context of that key verse is not only, or mainly, K’riat Sh’ma, but Talmud Torah]

1. When faced with Talmud Torah or another Mitzvah, we perform that other Mitzvah;

2. Talmud Torah is greater than all other Mitzvot;

3. If we don’t interrupt another Mitzvah in order to involve ourselves in Talmud Torah, how much more so that we do not interrupt one Mitzvah for another (not Talmud Torah) Mitzvah!

There is strong support for this proposal, that the entire rule of Ha’Osek b’Mitzvah is rooted in the Mitzvah of Talmud Torah. Talmud Torah is a Mitzvah which, by its very nature, demands a high level of concentration and single-mindedness. It is for that reason, it seems, that the Torah exempted us from learning when there is another Mitzvah which must be done and noone else can do it; because it is impossible to properly learn while doing something else. That explains why the the rule of Ha’Osek b’Mitzvah is guided by the level of concentration needed for a specific Mitzvah – both the one we are considering interrupting and the new one which has come along.


Halakha 5:

Q1: Why should we interrupt Talmud Torah to read K’riat Sh’ma – isn’t that what K’riat Sh’ma is – study?

A: All that and more. As I have pointed out several times in previous shiurim, K’riat Sh’ma is a “kiyyum” of Talmud Torah; but, in addition, it also comprises Kabbalat Ol Malkhut Shamayim (accepting God’s authority) – along with, as Rambam points out (1:2) “the [declaration of the] Unity of God, [the Mitzvah of] loving Him and [the Mitzvah of] studying Him (=his Torah).” We must note the opinion of R. Shim’on b. Yohai (JT Berakhot 1:2) that we do not interrupt study to recite K’riat Sh’ma – the argument provided there is that “Zeh Shinun veZeh Shinun” – both Talmud Torah and K’riat Sh’ma are learning – “v’ein m’vat’lin shinun mip’nei shinun” – we do not leave one learning for another. The Gemara goes on to qualify that this only holds true for R. Shim’on and his colleagues, whose learning was so constant and passionate that any part of learning was as valuable as K’riat Sh’ma.

Q2: Once he has interrupted his studying, why does Rambam need to to inform us that he also recites the B’rakhot?

A: We might think that we only interrupt study to recite K’riat Sh’ma because it is also learning (plus those other components mentioned); however, the B’rakhot, which do not comprise a kiyyum (fulfillment) of Talmud Torah, might not be worthy of interruption. In addition, if he interrupts study to recite K’riat Sh’ma, he remains in the context of study; while reciting it with the B’rakhot moves him from a pure study mode into a worship mode.

Q3: What is the definition of *Tzor’khei Rabim* in this context?

A: It likely means any “public need”; whether it is a court session (see BT Shabbat 11a – the example provided is when the court is in session to decide about intercalation of the year) or general public needs (e.g. raising funds for the needy). See MT Yom Tov 7:10-12 for a list of Tzor’khei Rabim. (for those who do not have access to the text, it basically includes public needs, such as clearing roads; along with all ritual and civil religious actions)

Q4: Why do *Tzor’khei Rabim* “outweigh” K’riat Sh’ma?

A: Here we utilize the general rule of “Ha’Osek beMitzvah Patur min haMitzvah” (see shiur above).

Art Werschulz (): Even more to the point, why do tzor’khei rabim outweigh talmud torah? After all, one must interrupt talmud torah for kriat sh’ma, but not tzor’khei rabim. Possible answer: There are presumably many people learning Torah at any given time. OTOH, the person engaged in a particular community need might be the only person capable of fulfilling said need.

Q5: How much time needs to be left in order to read K’riat Sh’ma – the “ideal” time or any time?

A: It would seem reasonable that if the Tzor’khei Rabim were completed before three hours into the day, which is the end of “Z’man K’riat Sh’ma”, that he should still read. However, note the approach of the Kessef Mishneh, that Rambam holds that K’riat Sh’ma of daytime essentially may be said all day; although that has no other practical application, it may be significant here; we may hold that if the public concerns were completed before the end of the day, that he should still say K’riat Sh’ma.

Halakha 6:

Q6: Where did Rambam get this list of activities?

A: The Mishna in Shabbat (1:2) states: “A person should not sit in front of the barber near the time for Minchah (the afternoon Tefillah) until he prays; [similarly] he should not enter a bathhouse or a tannery, begin a meal or a court case; however, if he began [any of these], he need not interrupt (the activity in order to pray). We interrupt [such activities] for K’riat Sh’ma but not for Tefillah.” This is most probably the source for Rambam’s list. See the answer to the next question for a fuller treatment of this passage.

Q7: Why wouldn’t someone have to interrupt these (non-obligatory) activities in order to read K’riat Sh’ma?

A: Commenting on the passage quoted above, the Gemara in Sukkah (38a) distinguishes between a situation where there is enough time after completing the activity to fulfill the Mitzva (K’riat Sh’ma or, in the case of the Gemara there, shaking Lulav) and where there is not enough time. If there is enough time, we need not interrupt the activity (although it is laudatory to do so); it may be completed before doing the Mitzvah in questions. If there is not enough time, we do need to interrupt the activity to perform the Mitzvah.

Rambam may be following this line of reasoning; that if there is enough time, he need not interrupt. In that case, the first clause of the Halakha is referring to a situation where there is definitely enough time to complete the activity (e.g. haircut) and then read K’riat Sh’ma in time.

The Gemara then draws another distinction – between Mitzvot which are d’Orayta (mandated by the Torah) and those that are d’Rabanan (Rabbinically ordained). RAN (18b in Rif pages) summarizes the results of the sugya as follows:

(a) regarding a Mitzva mid’Rabanan (like Tefillah), even if he began the meal (for example) after the time for that Mitzvah had arrived, he need not interrupt the meal, as long as there will be enough time after the meal to perform the Mitzvah on time;

(b) regarding a Mitzvah mid’Orayta (like K’riat Sh’ma), if he began the meal after the time for the Mitzvah arrived, he must interrupt immediately (as soon as he realizes) and perform the Mitzvah, even if there is enough time to finish eating and perform the Mitzvah on time. However, if he began the meal “legally” – i.e. before the time of the Mitzvah had arrived, he need not interrupt the meal, even to perform a Mitzvah d’Orayta – as long as there is sufficient time to finish eating and perform the Mitzvah on time.

We can then posit that Rambam accepts this distinction (staring “legally” or starting “late”), and that our Halakha is relating to a situation where he began the activity before the time for K’riat Sh’ma (actually, before 1/2 hour before K’riat Sh’ma time – this will be discussed later.)

Q8: What is the basis for the dispute between Rambam and RABD?

A: It may be that RABD does not accept the distinction between starting early or late; in any case, regarding a Mitzvah d’Orayta, one must interrupt immediately; alternatively, RABD may be understanding that Rambam is not making that distinction and that he would allow you to continue the meal even if you started late. One other option – he could understand the Gemara in Sukkah that the ultimate distinction is between Mitzvot d’Orayta (always interrupt) and those which are d’Rabanan (depends on if there is enough time left).

Q9: What is the “hint” in Berakhot, alluded to by RABD?

A: As we mentioned in a few earlier shiurim, R. Yehuda haNassi interrupted his teaching to read K’riat Sh’ma – the first verse. As outlined in those shiurim, this is a strong proof for those who hold that only the first verse is d’Orayta. Here we see that even if he began the activity (in this case, teaching) “legally”, he still must interrupt for a Mitzvah d’Orayta. If this is the correct identification of RABD’s Berakhot hint, then this only relates to interrupting for a d’Orayta regardless of when you started; it also presents the RABD-Rambam dispute in light of the first answer, Q3 above.


The RAN, quoted above in Q2, distinguishes between Mitzvot d’Orayta and d’Rabanan in re: interrupting a non-permissible activity (e.g. a meal) – i.e. one that was begun after the time for that Mitzvah arrived.

There are three ways to understand this distinction:

(a) “Weight” – we generally relate to Mitzvot from the Torah with more severity than those ordained by the Rabbis; for example, a “Safek” (case of doubt) is decided to the stringent side of the question if it involves a law from the Torah; it is decided to the lenient side of the doubt in case it involves a Rabbinic law. Therefore, as long as (under normal circumstances), there will be enough time to fulfill the Mitzva, we are not bothered by your putting it off. A “twist” on this is to incorporate the “Safek” issue here: Since we are not 100% sure that we will complete the activity in time and be able to perform the Mitzvah properly, let us judge the situation as a “Safek” – if it involves a Mitzvah from the Torah, we are stringent and interrupt; if it is Rabbinic, we need not interrupt.

(b) “Legislation” – we could posit that the Rabbis, when they mandated certain Mitzvot (e.g. Lulav during the rest of Sukkot after the first day), never legislated that it should necessitate interruption; whereas Mitzvot from the Torah carry this character with them.

(c) “Nature of the Concern” – perhaps the distinction begins from a different issue: We don’t interrupt for a Rabbinic Mitzvah, because our only concern is that the Mitzvah be fulfilled – “to fulfill the words of the Rabbis”; as long as there is sufficient time, we need not interrupt our [mundane] activity for the Mitzvah. Contradistinctively, when the time to perform a Mitzvah from the Torah (“the word of God”) has arrived, it is unseemly to get involved in another activity and this involvement itself constitutes a “slap” at the Torah; the obligation to interrupt the activity is a response to this wrong-headed prioritization – putting your own needs before your responsibility to God’s commands.

Rambam, Copyright (c) 1999 Project Genesis, Inc.