Then the LORD said to Moses,
"Dedicate to me all the firstborn sons of Israel and every firstborn male animal. They are mine."
So Moses said to the people, "This is a day to remember forever -- the day you left Egypt, the place of your slavery. For the LORD has brought you out by his mighty power. (Remember, you are not to use any yeast.)
This day in early spring will be the anniversary of your exodus.
References for Exodus 13:4
You must celebrate this day when the LORD brings you into the land of the Canaanites, Hittites, Amorites, Hivites, and Jebusites. This is the land he swore to give your ancestors -- a land flowing with milk and honey.
For seven days you will eat only bread without yeast. Then on the seventh day, you will celebrate a great feast to the LORD.
Eat only bread without yeast during those seven days. In fact, there must be no yeast in your homes or anywhere within the borders of your land during this time."
"During these festival days each year, you must explain to your children why you are celebrating. Say to them, 'This is a celebration of what the LORD did for us when we left Egypt.'