The Lord told the children of Israel in preparation for the tenth plague on Egypt, that,
“Seven days you shall eat unleavened bread. On the first day you shall remove leaven from your houses. For whoever eats leavened bread from the first day until the seventh day, that person shall be cut off from Israel. On the first day there shall be a holy convocation, and on the seventh day there shall be a holy convocation for you. No manner of work shall be done on them; but that which everyone must eat—that only may be prepared by you” (Exodus 12.15, 16).
This occurrence of seven is not incidental, but part of the plan of God and His use of the number 7.
He had a specific reason for the Israelites that they eat unleavened bread for seven days. He explained further,
“For seven days no leaven shall be found in your houses, since whoever eats what is leavened, that same person shall be cut off from the congregation of Israel, whether he is a stranger or a native of the land” (Exodus 12.19).
The Feast of Unleaven was connected with the Passover Feast when the children of Israel remembered how the Lord delivered Israel out of Egypt, that they had to leave in a hurry, so much so that they could not wait for bread to rise, but they took the dough without the yeast in it, and got out of Egypt.
After the Lord destroyed the firstborn of the Egyptians, and Israel started to move out of Egypt, Moses reminded them,
“Seven days you shall eat unleavened bread, and on the seventh day there shall be a feast to the LORD. Unleavened bread shall be eaten seven days. And no leavened bread shall be seen among you, nor shall leaven be seen among you in all your quarters (Exodus 13.6, 7).
Thus, the seven days of eating unleavened bread reminded them of the hasty departure out of Egyptian slavery.
Likewise, on the first day of the week, Christians eat the same unleavened bread of the Passover, because Jesus is our Passover (1Co 5.7), for by His blood we escape eternal destruction, and we eat the bread to remember the great sacrifice that He made for us.