Exodus 12:17-18

You shall also observe the Feast of Unleavened Bread, because on this very day I brought your hosts [grouped according to tribal armies] out of the land of Egypt; therefore you shall observe this day throughout your generations as an ordinance forever. In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, [and continue] until the twenty-first day of the month at evening.