Instructions about the feast of unleavened bread are given as the Israelites are preparing to leave the slavery in Egypt.
The feast is to be a memorial of their escape from Egypt:
17 You shall observe the feast of unleavened bread; for in this same day I have brought your armies out of the land of Egypt. Therefore you shall observe this day throughout your generations by an ordinance forever. 18 In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, until the twenty first day of the month at evening. 19 There shall be no yeast found in your houses for seven days, for whoever eats that which is leavened, that soul shall be cut off from the congregation of Israel, whether he is a foreigner, or one who is born in the land. 20 You shall eat nothing leavened. In all your habitations you shall eat unleavened bread.’ ”
When the law is given to Moses, the feast of unleavened bread is included:
14 “You shall observe a feast to me three times a year. 15 You shall observe the feast of unleavened bread. Seven days you shall eat unleavened bread, as I commanded you, at the time appointed in the month Abib (for in it you came out of Egypt), and no one shall appear before me empty.
The book of Leviticus, which gives details of all aspects of the Israelites' worship, also gives instructions about the feast of unleavened bread:
4 “ ‘These are the set feasts of the LORD, even holy convocations, which you shall proclaim in their appointed season. 5 In the first month, on the fourteenth day of the month in the evening, is the LORD’s Passover. 6 On the fifteenth day of the same month is the feast of unleavened bread to the LORD. Seven days you shall eat unleavened bread. 7 In the first day you shall have a holy convocation. You shall do no regular work. 8 But you shall offer an offering made by fire to the LORD seven days. In the seventh day is a holy convocation. You shall do no regular work.’ ”
As they are about to go into the promised land, the Israelites are reminded to keep the feast of unleavened bread:
16 “ ‘In the first month, on the fourteenth day of the month, is the LORD’s Passover. 17 On the fifteenth day of this month shall be a feast. Unleavened bread shall be eaten for seven days.