To differentiate normally (not this year, as Yom Tov falls on Shabbos) between Shabbos mimochras hashabbos, the day after Yom Tov for sefira and a regular Shabbos which is called Shabbos Hagadol, the great Shabbos being a complete rest. The Tur O’CH 430 says that it is called so to commemorate the miracle that happened on Shabbos where the Jews took the Pesach sacrifice and tied it to the bed post telling the Mitzryim that it will be a sacrifice which was their god and they could not do anything to them. As this happened on a Shabbos, it strengthened the holiness of shabbos which protected them thereby realizing the greatness of Shabbos.