Shabbat Hagadol

Shabbat Hagadol is the day upon which, historically, the Jewish nation in Egypt took the sheep, in full view of the Egyptians, that were to going be used for the Korbon Pesach. Since that day was Shabbat, we mark this event also on the Shabbat preceding Pesach. The classic question is: why would this event not be marked in the normal way through the calendar date?

Nishma's Rabbi Hecht discusses this issue in http://www.nishma.org/articles/insight/insight5767-23.htm