Shabbat Hagadol

Q. Why is the Shabbat before Pesach called “Shabbat HaGadol”, “The Great Sabbath”?

A. The explanations include the following:

* Great (“gadol”) events happened in the lead-up to Pesach.
* The haftarah (from Malachi) refers to “the great and awesome (“gadol v’nora”) day of the Lord” (Mal. 3:22-24).
* In days when rabbis did not give regular sermons, the great scholar (“gadol”) of the community gave a discourse on that day, which made it “the Sabbath of the Great One”.
* Adding sections from the Haggadah to the afternoon service, and listening to the extensive discourse, made the day “gadol” (longer than usual).
* The name might originally have been “Shabbat Haggadah”, later corrupted to become “Shabbat HaGadol”.
http://www.oztorah.com/2007/03/the-name-shabbat-hagadol-ask-the-rabbi/