The Feasts of Passover commemorates the deliverance of the Hebrew people from Egypt as described in the book of Exodus. It is celebrated with worship and a meal on the fourteenth day of the month called Nisan, which is the first month of the Jewish year. The time... More and Unleavened Bread are reinterpreted with respect to Israel’s experience of deliverance at the Red Sea and the centralization of worship in Jerusalem.
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, Judean king noted for his reforms of Israel's worship in the time of Jeremiah More 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.