Passover is the defining moment in the history of the descendents of Jacob (Israel), son of Isaac, son of Abraham.

After hundreds of years of slavery in Egypt, God delivered his people by striking the land with plagues. To protect them from the effects of the deadly final plague, God gave them detailed instructions -- instructions that initiated a lasting celebration for generations to come 1 .

A lamb feast was shared by everyone. But as the lamb was slaughtered, its blood was applied to the posts and lintel of their doors. And they were to eat the feast prepared for sudden departure --  with sandals and belts fastened.
The Passover Lamb is to be one year old and "without blemish".
And this "sacrifice" was to be entirely eaten! 
To experience freedom from slavery, the people needed to be protected from death, and to depart to start a new life.