There are many explanations as to why the Sabbath before Passover is called Shabbat HaGadol.
Excerpted with permission from Every Person’s Guide to Passover (Jason Aronson, Inc).
The Sabbath immediately preceding Passover is called Shabbat HaGadol, the great Sabbath. According to tradition, the 10th of Nisan in the year of the exodus was a Saturday. It was considered a great event, in fact a miracle, that the Israelites could on that day select a lamb for sacrifice without being molested by their Egyptian masters, who, at other times, would have stoned them for such daring (Code of Jewish Law, Orach Chayyim 430: 1).
Another possible reason for the name is that the haftarah (Malachi 3:4-24), the prophetic portion, speaks of the "great day" of God on which the Messiah will appear. A novel explanation for the name of Shabbat HaGadol is that the people used to return from the synagogue later than usual on this Sabbath because of the unusually long sermon that was customary on this day.
The custom of reciting the Haggadah in the afternoon of Shabbat HaGadol was designed to familiarize the people with its contents in preparation for the Seder service that week (Code of Jewish Law, Orach Chayyim 430).