A person who wants to fast should undertake to do so at the afternoon service of a preceding day (see 562:7-11), preferably after he finishes reciting SHEMONEH ESREH (see 562:6). Undertaking it at a different time, even verbally, is not valid (563:1), but a person may fast because of a bad dream even if he did not undertake to do so before sunset (see 562:5). A fast begins at dawn (see 562:7); if a person falls asleep and wakes up before dawn, he must not eat unless he had planned to do so (see 564:1). Ordinarily, a fast lasts until dark; on cases where it may be ended earlier see 562:1-4. When a public fast is decreed, everyone must also undertake it individually (see 562:12;568:6;574:1-5); and a person should go hungry when others are hungry (see 575:4).
Individuals who are fasting recite “Answer us…” in the 16th blessing of the afternoon SHEMONEH ESREH (see 565:1,3); on what to do if it was omitted see 565:2. When the leader (who must be fasting; 566:5) repeats SHEMONEH ESREH in the morning and afternoon on a public fast, he recites it as a separate blessing, ending “…Who answers in time of trouble”, after the seventh blessing (566:1,3). On the Torah reading in the morning and afternoon on a public fast, and the reading from the Prophets in the afternoon, see 566:1-2,6. On the prayers and SELICHOS that an individual recites on a private fast see 565:4-5; on the SELICHOS that are recited on a public fast see 566:4,7; on the priestly blessing see 566:8.
A person who is fasting should behave seriously and abstemiously (see 568:12). On tasting food or washing the mouth out during a fast see 567:1-3. A person who accidentally eats on a fast should finish the fast (see also 574:3) and should also fast on another day in its place; see 568:1. A person who undertakes to fast without specifying the day may change his mind after he has started to fast, and fast on another day instead; see 568:2. On cases in which substitutions are (or are not) allowed see 568:3-4,10-11; on conditional acceptance of a fast see 562:13. A person may fast because of a bad dream even on days when fasting is not allowed, but he must then also fast on another day as a penitence for fasting at an improper time (568:5). On fasts on the anniversary of a parent’s death see 568:7-9. If an individual is fasting for a reason that becomes invalid he must complete his fast; but if he fasted in error he need not complete it (569:1-2; see also 575:11-13 for the case of a public fast).
It is improper for a person to boast that he is fasting (565:6). A person who is able to fast and does so is called holy; if he fasts when he is unable to do so, he is considered a sinner (571:1). Scholars should not fast if it will interfere with their studies, but they must fast when the community does (571:2). A person should not fast at a time of troubles if it interferes with his ability to cope with them, but he should promise to fast after the troubles are over (571:3). Fasts should not be decreed on New Moons, intermediate days of holidays, CHANUKAH, or PURIM, but if they were decreed on those days, they should be observed; see 572:1-3. On fasts that are undertaken by individuals on such days see 570:1-3. The Scroll of Fasts, which listed many other days on which fasting was not allowed, is no longer in effect; see 573:1.
In the land of Israel (see also 575:9-10) special fasts were decreed when the rains did not come on time; see 575:1-11. On the order of prayers on these fasts see 579:1-3. Fasts are also decreed because of invading armies (see 576:1); epidemics (see 576:2-3,5); storms and earthquakes (see 576:4); floods (see 576:11;577:1); attacks by animals (see 576:6-7); or problems with crops (see 576:8-10). On local problems see 576:12-13; on multiple problems see 576:15 and 577:1; on practices associated with these fasts see 576:14-16. Individuals should also fast when they encounter troubles; see 578:1. Fasting is also appropriate on certain dates when historical disasters occurred, and on Mondays and Thursdays; see 580:1-3.
Shulchan Aruch, Copyright (c) 2000 Project Genesis, Inc.