Why do the children of Israel celebrate the seventh night of Pesaḥ (referred to by Rav Sa’adya Gaon as "Ḥag HaShmada" – the “Festival of Destruction”)?

Yehuda HaKohen teaches that in addition to commemorating the smashing of Egypt’s military at the Sea of Reeds, the 21st of Nisan also marks something more significant to Israel’s development – the climax of our miraculous national birth and the attainment of collective prophecy.