Exodus 13:8-10

And thou shalt tell thy son in that day, saying: This is what the Lord did to me when I came forth out of Egypt.

And it shall be as a sign in thy hand, and as a memorial before thy eyes; and that the law of the Lord be always in thy mouth, for with a strong hand the Lord hath brought thee out of the land of Egypt.

Thou shalt keep this observance at the set time from days to days.