A batch of starter dough was used to leaven the next batch of dough and time was needed for the dough to rise before baking. The new dough was in the mixing bowls when Pharaoh sent the Israelites away; therefore, it was not leavened and had to be baked for food while traveling. Once they settled into the promised land, the Israelites were to eat unleavened bread for seven days, in remembrance, that the Lord took them out of Egypt in haste.
Both the feast of the passover lamb and the feast of unleavened bread would be from this point on a memorial of redemption in the lives of the Israelite. The Lord passed-over the entire congregation on the night of judgment in Egypt and he led the Israelites out of Egypt as an entire congregation; therefore, it was observed with an assembly at the beginning and end of the week to celebrate the Lord’s passover of the sons of Israel and their exodus from Egypt.
When the Egyptians saw that the first born had died in every household in Egypt, they rushed to send away the sons of Israel least they all die. The Egyptians lost great wealth when the Israelites departed. The sons of Israel took with them silver and gold from the land of Egypt. A very large assembly departed Egypt that night and traveled from Rameses to Succoth. This all occurred four hundred years after Jacob and his sons entered the land of Egypt. Thus, the word of the Lord was fulfilled, which he spoke to Jacob, in visions of the night, while he slept at Beersheba, in the land of Canaan, before the sons of Israel took their father and their little ones and their wives to Egypt. (cf.Gn.46:1-7)