The Tur (O.C. 430) writes that the reason why the Shabbos prior to Pesach is referred to as Shabbos HaGadol is because of the great miracle that occurred to the Jewish People in Egypt. The Sar Shalom from Belz asks, does not HaShem performs miracles for us every second of the day, and we find (Yoma 69b) that the Anshei Kneses HaGedolah returned the Crown to its glory when they reinstituted the recital of the words Gadol and Nora in Shemone Esrei. The reason why they reinstituted this is because they said, “these are HaShem’s strengths and awesomeness, that a nation like the Jewish People can exist amongst the nations of the world.” This being the case, why is it that specifically on this Shabbos we refer to the Shabbos as Shabbos HaGadol because of the miracle that occurred?
The Sar Shalom answers that the miracles that HaShem performs for us on a constant basis are considered hidden miracles, but the miracles that HaShem performed for us in Egypt were deemed to be open miracles, as it was evident to all that HaShem is our Father and King. For this reason a miracle is called nes, which connotes something that is high, as it is said (Yeshaya 49:22) viel amim arim nisi, and I will hoist my banner towards people.
It is written in the sefarim that the three festivals correspond to Gadol, Gibor, and Nora, and Pesach corresponds to Gadol. It is well-known that on the Shabbos that precedes the festival, the power of the festival is aroused. Thus, on the Shabbos that precedes Pesach, the “great” power of Pesach is aroused, and for this reason, this Shabbos is referred to as Shabbos HaGadol.