On the Shabbat before the Exodus—the 10th of Nisan in that year—the Egyptian firstborn, who held important positions in the priesthood and government, fought a bloody battle against Pharaoh's troops in an effort to secure the Israelites' liberation and prevent the plague of the Death of the Firstborn. This great miracle is commemorated every year on the Shabbat before Passover, which is called Shabbat HaGadol—"The Great Shabbat.".
Other scholars believe it is called this because it was the day the city rabbi gave the final instructions to the people regarding the laws of Passover. It was customary for the entire town to attend the synagogue to hear him speak, so it was considered the Shabbat on which the city's "great one" spoke; hence, it was called "The Shabbat of the Great One.".
Because it is the Shabbat before Passover, it is customary to recite a part of the Haggadah (the liturgy of Passover night) as preparation for that night.