Exodus 13:8-10

And thou shalt shew thy son in that day, saying, This is done because of that which the LORD did unto me when I came forth out of Egypt.

And it shall be for a sign unto thee upon thine hand, and for a memorial between thine eyes, that the LORD'S law may be in thy mouth: for with a strong hand hath the LORD brought thee out of Egypt.

Thou shalt therefore keep this ordinance in his season from year to year.