More than once the Lord told Israel that He wanted them to observe the Feast of Unleavened Bread, during which they would not eat leavened bread for seven days, as it is written,

“The Feast of Unleavened Bread you shall keep. Seven days you shall eat unleavened bread, as I commanded you, in the appointed time of the month of Abib; for in the month of Abib you came out from Egypt” (Exodus 34.18).

Eating unleavened bread for seven days would get across to them that they had come out of Egypt in a hurry, and later generations would also get the point.

Seven days would do the job of reminding.