Exodus records that God sent ten plagues against the Egyptians because Pharaoh repeatedly didn’t allow Moses to lead the nation of Israel out of slavery. First, the Nile River turned to blood (1), then frogs covered the land (2), followed by infestations of lice (3) and flies (4). Disease then struck livestock (5), with boils breaking out on people as well (6). Severe hailstorms (7) led to swarms of locusts that devastated crops (8), followed by three days of darkness (9). The final plague was the death of every firstborn male in Egypt (10). During each plague, only the Egyptians and their property were affected, not Israel. But to prevent the last and most severe plague from striking Israel’s sons, Moses instructed the entire nation to sacrifice a lamb and wipe its blood on their doorposts the night before the angel of death was to come. The blood would be a sign for the angel to pass over the house, sparing the firstborn sons of Israel. That night, after finding dead sons in every Egyptian house, Pharaoh finally let Israel go.
Thus the tradition of celebrating the Passover Feast began, to commemorate when the angel of death passed over Israel’s homes. It was celebrated in the spring, and all of Israel’s men were required to gather in Jerusalem to feast and rest for a week. On the first night, a flawless lamb was sacrificed and eaten, and for the rest of the week, all bread had to be flat and unleavened. In fact, any form of yeast or leavening agent was prohibited during the feast, since, in their haste to leave Egypt centuries prior, Israel had no time to include yeast in their bread and wait for it to rise. The Passover was therefore also called the Feast of the Unleavened.