The Shabbat before Pesach is known as Hagadol, “the Great Sabbath”. Many explanations have been offered for this title. Here are four summarised from Rabbi Enkin of Ramat Beit Shemesh.
The most common explanation relates to the Passover offering that the Jewish people were commanded to prepare. On the Shabbat prior to the Exodus, the 10th of Nisan, they were commanded to take a sheep and tie it to their beds. The Egyptians worshipped sheep. When they saw what was being done, they became enraged and demanded an explanation for this sacrilege. The Jews explained that they were going to slaughter the sheep as an offering to Hashem. Under normal circumstances, a deadly reprisal would have followed, yet, not a single Jew was harmed. In memory of this miracle, the Shabbat before Pesach became Shabbat Hagadol.
Secondly, as a result of being steeped in Egyptian society for so many years, many Jews adopted the Egyptian religion and had begun worshiping sheep themselves. However, the anticipation of the impending Exodus influenced many to turn from their idolatrous ways. In order to recall the mass repentance on the Shabbat before the Exodus, it was designated as Hagadol. So too, the nation as a whole affirmed their acceptance of Hashem’s authority on this day. The epithet Hagadol in this context refers to Hashem, who is truly ‘Gadol’. Another reason for the Shabbat Hagadol designation is that when the Jews told the Egyptians why they were tying sheep to their beds, they also took the opportunity to notify them of the upcoming tenth and final plague, the death of all the firstborn. When the firstborn Egyptians heard this, they demanded Pharaoh release the Jews immediately in order to save their lives. When Pharaoh refused, the firstborn went on a rampage killing many of their fellow Egyptians. This remarkable turn of events also warranted the designation of Gadol.
Rabbi Aaron of Belz offers another idea. He says that the Shabbat before Pesach was the first Shabbat that the Jews were commanded to observe. Although the Jews had previously rested on Shabbat while slaves to Pharaoh, it was only on Shabbat Hagadol that it became a Divine mitzvah. Performing a mitzvah when specifically commanded to do so by Hashem is ‘Gadol’, greater, than doing it for other reasons.