The Shulchan Aruch (Orech Chayim 430) states that “The Shabbos before Pesach is called *Shabbas HaGadol,’ *The Great Shabbos,” because of the miracle that occurred on this day.” The Ram”a adds to this that “the custom is to say, by Mincha, the Hagadah, beginning with the passage of Avadim HaYinu and ending with the passage of “Lechaper al kol avonoseinu.”
The Mishna Berura explains that in the year the nation of Israel departed from Egypt, the 10th day of Nissan was on Shabbos. Everyone in the nation took on that day a sheep for his Paschal offering and tied it to his bedpost. The Egyptians saw this and queried as to what they were doing. The people answered they were going to slaughter this sheep as a Paschal offering, according to the commandment of G-d. The Egyptians were not able to respond to the information that their god was to be slaughtered. Because this 10th day of Nissan was on Shabbos, the Shabbos before Pesach has since been called Shabbos HaGadol. Furthermore, since the beginning of the miracles associated with exodus began with this day, we have the tradition to read the Hagadah Shabbos afternoon, even if Shabbos HaGadol occurs on Pesach eve (as it does this year).
Another reason offered for why we read the Hagadah on the Shabbos before Pesach concerns the laws regarding special prayers for the holidays. The Shulchan Aruch (Orech Chayim 100) writes that regarding the prayers for the holidays and Rosh Chodesh, people should review and prepare the prayers before they actually pray, so that when they actually do pray they are fluent in the text of the prayer. Similarly, the Maharshal writes (Teshuva 88), one should review the Hagadah before Pesach, so that each person can recite the Hagadah the night of Pesach with a greater sense of fluency and understanding.
The Vilna Gaon has a general objection to this custom. We read in the Hagadah that one might think that the commandment of relating the story of the exodus begins with the start of the month of Nissan. However, the connotation of the scriptural verse is that this obligation only exists when the Matzo and Maror are before us at the Seder table. If we are to wait to the time of the Seder for the recitation of the events as contained in the Hagadah, how then can we recite the Hagadah on the Shabbos before Pesach?
The Hagadah Agudas Aizov offers a resolution of the Vilna Gaon’s contradiction. The verse which indicates that we recite the Hagadah only at the Seder refers specifically to the commandment that we must relate these events to our children. At the Seder, the children have to be motivated to stay interested. The children need to have their curiosity piqued. If the contents of the Seder night are laid to them in advance, they will lose interest and not focus on the important message that we must impart to them at the Seder. Therefore, we are not supposed to relate the story of the Exodus, as contained in the Hagadah, until we have arrived at the Seder. Adults, however, are able to appreciate new nuances and deeper understandings each time they read the Hagadah. As adults can gain new insight with each time they review the Hagadah, it is clear that reading the Hagadah before the Seder night will not cause them to lose interest come the actual Seder night. In fact, the opposite should occur: the insights gleaned in the review of the Hagadah before Pesach will enrich the Seder experience.
It is clear from this custom that the recitation of the Hagadah is of utmost importance on Pesach. We should review it beforehand so that we are familiar with its contents. We have to relate the Hagadah to our children in a way that will best impress upon them the important message that in contains. And we, the adults, have to appreciate the Hagadah anew every year.
(adapted from Minhag Yisroel Torah)