Shabbos HaGadol is the Shabbos that comes before Pesach. The main reason that it is called Shabbos HaGadol is because of the great miracle that occurred then. The Jewish people were ordered to tie the sheep – which they were going to offer as a Korban Pesach – to their bedpost. The Egyptians, who worshipped the sheep, could only look on in dismay and said nothing. This was a tremendous miracle.