"In the first month, on the fourteenth day of the month at twilight is the Lord's Passover." (Leviticus 23:5)
The original Passover is told in the book of Exodus. You may know it as all the plagues and when Moses parted the Red Sea. Well, did you know that we are commanded in Exodus to keep a memorial to remember this great occasion forever?
"The blood shall be a sign for you, on the houses where you are. And when I see the blood, I will pass over you, and no plague will befall you to destroy you, when I strike the land of Egypt. This day shall be for you a memorial day, and you shall keep it as a feast to the Lord; throughout your generations, as a statute forever, you shall keep it as a feast." (Exodus 12:13-14)