The Passover was originally commanded while Israel was in slavery in Egypt (Exodus 12:1-16). Exodus 12:6 says concerning the lamb that they were to designate on the 10th day of the month Abib, “Take care of them until the fourteenth day of the month, when all the people of the community of Israel must slaughter them at twilight.” This twilight was at the beginning of the day, after the 13th ended.
Exodus 12:17-20 speaks of the Feast of the Unleavened Bread commanding that it was to start “on the evening of the fourteenth day until the evening of the twenty-first day (of Abib). When the Israelites reached Mount Sinai the Lord changed the way the Passover was to be celebrated when they reached the promised land.
Leviticus 23:4-8 also confirms they are separate days, with Passover starting at the beginning of the day and the Feast of Unleavened Bread starting at the end of the same day. “‘These are the LORD’s appointed feasts, the sacred assemblies you are to proclaim at their appointed times: The LORD’s Passover begins at twilight on the fourteenth day of the first month. On the fifteenth day of that month the LORD’s Feast of Unleavened Bread begins; for seven days you must eat bread made without yeast. On the first day hold a sacred assembly and do no regular work. For seven days present an offering made to the LORD by fire. And on the seventh day hold a sacred assembly and do no regular work.'”
Deuteronomy 16:1-8, 16-17 is a repeat of what is already defined in Exodus and Leviticus. One addition is, “You must not sacrifice the Passover in any town the LORD your God gives you except in the place he will choose as a dwelling for his Name. There you must sacrifice the Passover in the evening, when the sun goes down, on the anniversary of your departure…” This is an addition from when they were in Egypt and at Mount Sinai. The place became first Shiloh, where the tent of meeting was placed by Joshua (Joshua 18:1, 8, 10, 19:51, 21:2, 22:12; Judges 18:31, 21:19; 1 Samuel 1:3, 9, 24, 3:21, 4:3-4, 14:3; Psalm 78:60; Jeremiah 7:12-14) until David moved it to Jerusalem and his son Solomon built the Temple (1 Kings 8:4; 1 Chronicles 6:32, 15:25-29). Jesus would make another change during the Last Supper.
As time went on, the Israelites often called Passover “The Feast of Unleavened Bread,” when in fact they are separate holy days.
