Exodus 13:10

Overview - Exodus 13
1	The firstborn are sanctified to God.
3	The memorial of the passover is commanded.
11	The firstlings of man and beast are set apart.
17	The Israelites go out of Egypt, and carry Joseph's bones with them.
20	They come to Etham.
21	God guides them by a pillar of a cloud, and a pillar of fire.

Exodus 13:10  (King James Version)
Thou shalt therefore keep this ordinance in his season from year to year.