Passover was originally a pre-Mosaic rite in which shepherds would celebrate the lambing of their flocks by sacrificing a lamb, smearing its blood on their tent flaps, and dancing a hopping, skipping dance to ward off evil. The farmers would celebrate the barley harvest by cleaning out all the old leaven (similar to our sourdough) and beginning the process of leavening the flour with lumps of fermented dough all over again. At some time in the history of Israel these two ways of welcoming the lambing of the flock and the barley harvest in the spring became joined in celebration of the birth of Israel in the exodus. When this occurred is still a matter of dispute. Many assume that Josiah's centralization of the Passover in 621 B.C.E., the first to be so celebrated "since the days of the judges" (2 Kings 23:21-23), makes the most sense. Conceivably, Josiah revived an older practice.
The Passover celebration was first to be transformed. The blood of the sacrificed lamb smeared upon the doorposts of houses was seen as warding off the "angel of death" who "passed over" homes thus marked, during the tenth plague of Egypt (Exodus 11-12). The verb "pass over," the sacrificed lamb, and the hopping, skipping dance of the shepherds are all derivatives of the Hebrew verb pesah.
The Feast of Unleavened Bread also was transformed. Here, the reason for the eating of unleavened bread was not agricultural but theological. The Israelites had no time to let their bread rise at the Exodus; haste was required.
Thus the two separate festivals have been merged to celebrate the lambing of the flock, the barley harvest, the birth of the nation at the exodus, and the life of every firstborn son who was "redeemed" at the time of Israel's departure from Egypt.
Before the institution of Passover, the year had begun in the month of Tishri, in the autumn. Now, in commemoration of God's deliverance of the people, it begins in Abib (later, Nisan), in the springtime. Deuteronomy adds the requirement that the combined festival take place "at the place that the LORD will choose as a dwelling for his name" (16:2, 6, see also 7) in conformity with the principle of worship at the central shrine (12:5, 11, 14). Originally, Passover was celebrated by families in their homes.
16Observe the month* of Abib by keeping the passover to the Lord your God, for in the month of Abib the Lord your God brought you out of Egypt by night. 2You shall offer the passover sacrifice to the Lord your God, from the flock and the herd, at the place that the Lord will choose as a dwelling for his name. 3You must not eat with it anything leavened. For seven days you shall eat unleavened bread with itthe bread of afflictionbecause you came out of the land of Egypt in great haste, so that all the days of your life you may remember the day of your departure from the land of Egypt. 4No leaven shall be seen with you in all your territory for seven days; and none of the meat of what you slaughter on the evening of the first day shall remain until morning. 5You are not permitted to offer the passover sacrifice within any of your towns that the Lord your God is giving you. 6But at the place that the Lord your God will choose as a dwelling for his name, only there shall you offer the passover sacrifice, in the evening at sunset, the time of day when you departed from Egypt. 7You shall cook it and eat it at the place that the Lord your God will choose; the next morning you may go back to your tents. 8For six days you shall continue to eat unleavened bread, and on the seventh day there shall be a solemn assembly for the Lord your God, when you shall do no work.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org