The Shabbat before Pesach is called "Shabbat Hagadol" (the "Great Shabbat") for a number of reasons:
1) The primary event commemorated on this Shabbat is a great miracle which occurred on this day, several days before the Exodus. The Jewish people were commanded by Moses to take a lamb and tie it to their bedposts on Shabbat, the 10th day of Nissan, five days before they were to leave Egypt. When the Egyptians inquired by the Jews why they were buying lambs en masse, they were told that these lambs were intended for the Paschal Offering, which would be sacrificed in preparation of the Plague of the Firstborn. For some reason, this information rattled the Egyptian firstborn, who immediately insisted that Pharaoh grant the Jews the liberty they demanded. When Pharaoh refused their request, the Egyptian firstborn waged war with Pharaoh's army, and many Egyptians who were guilty of atrocities against the Jews were killed on that day.
2) Furthermore, on this day it was demonstrated that the Egyptians were powerless against the Jews. They must have been mightily peeved by the fact that the Jews were planning to slaughter lambs, an Egyptian deity -- but were incapable of doing anything to hamper their plans.
3) Some suggest that this Shabbat earned the title "Gadol," because it is the day when the rabbis traditionally deliver extensive lectures about the laws of Passover, and pontificate about the lessons to be learned from the holiday.
4) The Haftorah read in many communities on this Shabbat speaks of the coming of Moshiach, referring to the day of his arrival as the "yom Hashem hagadol v'hanora" -- the "great" and awesome day of the L-rd.
Click here for more about Shabbat Hagadol.
Have a Happy and Kosher Pesach!
Rabbi Baruch S. Davidson