The festival calendar in Deuteronomy 16 began as a short revision of the calendar in Exodus 23. As it was expanded to clarify and adjust its details, it merged its springtime Matzot festival with the Pesach offering, which was originally connected to the consecration of firstborn animals.
Reinhard G. Kratz
To mark the new year of grain and ensure the bountiful wheat harvest to come. But why do we remove all our chametz (leaven)?