The Shabbat before Pesach is known as Shabbat Hagadol שבת הגדול - "The Great Sabbath". While the phrase "shabbat hagadol" appears in other contexts earlier in Rabbinic literature (such as in the Retze section of Birkat Hamazon), it is first mentioned as the shabbat before Pesach in the works of Rashi.
For example, in his book Sefer Hapardes, Rashi begins his explanation of the reason for this title by writing:
People are accustomed to calling the shabbat before Pesach "Shabbat Hagadol", but they do not know what makes this shabbat greater than any other.
He then continues:
The Children of Israel went out of Egypt on a Thursday, as is recorded in Seder Olam. They prepared the lamb for the Pesach sacrifice on the previous shabbat, on the tenth of Nisan. When they were instructed to do so, they wondered: "If we sacrifice an animal which the Egyptians hold sacred, before their very eyes, they will surely stone us." But God told them: "Now you see the wondrous things which I will do for you." The Children of Israel thereupon each took a lamb and kept it for four days. When the Egyptians saw this, they wanted to rise up and take revenge, but they were stricken with all kinds of horrible afflictions and could do no harm to the Children of Israel. Because of the miracles which God performed on that day, the Shabbat before Pesach, it became known as Shabbat Hagadol.
This is a popular explanation, but it is possible to sense from his introduction that Rashi is giving one answer to a question that many people had asked over the years. And in fact, there are many other explanations given for the origin of the name.
Others say that the reason is found in the Haftara read on that day (according to the Levush only when that shabbat is the day immediately before Pesach), describing Messianic times, and ends with the verse from Malachi 3:23:
הִנֵּה אָנֹכִי שֹׁלֵחַ לָכֶם, אֵת אֵלִיָּה הַנָּבִיא--לִפְנֵי, בּוֹא יוֹם ה', הַגָּדוֹל, וְהַנּוֹרָא
Behold, I will send you Eliya the prophet before the coming of the great (hagadol) and terrifying day of God.
Just like other special shabbatot are named for their haftara (Shabbat Chazon, Shabbat Nachamu, Shabbat Shuva), according to this approach so is Shabbat Hagadol. Some point out that because it is named for the word, the day is called Shabbat Hagadol instead of Shabbat Gadol.
Others use the verse to answer the question why, if the word shabbat is feminine, why isn't the day called Shabbat Hagedola? However, this isn't such a serious question, as we find shabbat as a masculine noun in Biblical Hebrew - shabbat b'shabbato שבת בשבתו (Bamidbar 28:10, Yeshaya 66:23) and shomer shabbat mechalelo שומר שבת מחללו (Yeshaya 56:2). And later in Rabbinic Hebrew, shabbat appears in the masculine in the Amida prayer of Shaharit on shabbat - veyanuchu bo וינוחו בו, and in the greeting shabbat shalom u'mevorach שבת שלום ומבורך.
However, there are those that reject this explanation, since the other haftarot are all named for the first word read, and Shabbat Hagadol does not fit this pattern. Fishbane, in the JPS Haftarot points out that the connection between Shabbat Hagadol and the "great day" in the haftara is too obvious: certainly Rashi and others would have mentioned it had they read this haftara on that day. According to this article, both the Tur and the Levush claim that the haftara was chosen after the day had already been known as Shabbat Hagadol.
Another possible explanation (mentioned by the Shibolei Haleket) relates to the custom on that shabbat of the rabbi giving a long drasha (sermon) - maybe the longest of the year. This shabbat is compared to Yom Kippur, which is also called a "great fast" צומא רבה (Peah 7:4) because of the long prayers (not because of the length of the fast, which is the same as Tisha B'Av).. There are those that temper the cynicism of this approach by saying that it does refer to the sermon, but the day is called "great" because of the importance of the speech, the congregation or the rabbi, not as a complaint to its length. Others say that the custom of the drasha came after the name had been established, but it was originally called a long shabbat because many additional prayers were added on that day.
Other less familiar reasons given include:
- Just like a child becomes an adult (gadol) when he accepts the mitzvot (bar mitzva), so too did the Jews when they accepted their first mitzva.
- In the Torah, the omer offering is brought ממחרת השבת - the day following shabbat. According to Rabbinic tradition (and in opposition to the Sadducees and Karaites) the "shabbat" referred to in the verse is the first day of Pesach. So Shabbat Hagadol refers to the seventh day of the week, as compared to a lesser type of shabbat (in terms of prohibitions) on the first day of the holiday.
- According to a midrash, during their slavery in Egypt the Jews did not work on shabbat. However, immediately following shabbat they would need to return to work (there's a parallel expression in the Israeli army - "every shabbat has a motzei shabbat", since soldiers can't be punished on shabbat itself). However, on this shabbat, the Jews were no longer slaves, so they didn't need to fear returning to their labors.
- Some sources, particularly from Medieval Italian Jewry, seem to indicate that perhaps the Shabbat before every holiday was called "Shabbat Hagadol". However, it is possible that the name spread from the Shabbat before Pesach to the other holidays, instead of the other way around.
Although we stated that the name Shabbat Hagadol first appears as the shabbat before Pesach in Rashi's time, there are those (such as Zunz and later Safrai in the Haggadah of the Sages) who believe that it was probably called that going back much earlier. They note that in early Christian sources, such as John 19:31, we find mention of a "great sabbath". Therefore the term must have been used by Jews at the time. However, others reject this approach. For example, Sacha Stern in Calendar and Community: A History of the Jewish Calendar, 2nd Century BCE to 10th Century CE writes that the passage "is not relevant as it refers to the Sabbath following Passover" (along with other objections).
As usual, when we have so many explanations, the chance of any one of them being correct decreases. However, when it comes to Pesach - we have a tradition of asking many questions and studying as much as possible. So perhaps the origin of the name was not made clear so we could continue to learn about it every year...