Likutei Sichos, vol 27, Shabbos Hagadol, Rabbi Chaim Wolosow
https://www.torahrecordings.com/likutei-sichos/027/004_001 The Rebbe explains why the miracle remembered on Shabbos HaGadol is commemorated on the day of the week rather than on the 10th of Nisan. Unlike other miracles celebrated on calendar dates, this miracle revealed a deeper idea: the transformation of nature itself, when the Egyptian firstborn fought their own people.