http://revach.net/article.php?id=3612

Section: Shabbos   Category: Likras Shabbos
Shabbos HaGadol - Literally The Biggest Shabbos In History
Shabbos HaGadol. Why do we call it the big Shabbos?  The Chasam Sofer explains with the Tur that says that on Motza'ei Shabbos we don't say "Vihi No'am" if during the upcoming week there is a Yom Tov.  One of the reasons for this is because if Shabbos is not followed by six work days the Kedushas Shabbos does not leave.

Up until Yetzias Mitzrayim every Shabbos had 24 hours of Kedushas Shabbos.  The Shabbos before Pesach was the first time in the history of the world that Kedushas Shabbos was extended beyond its usual time slot.  Therefore the people called it Shabbos HaGadol and we continue to recognize this special extra Kedusha until today.