Read Exodus 13
Although I have read and taught these books many times I am amazed that every time I go back I find something I missed before. Many people believe, and I used to believe, that this feast was the same as Passover. It actually begins the day after Passover. Passover originally was only one day, the feast of unleavened bread is a week, it begins the day after Passover. Here in Exodus we see them as two separate feasts but in Deuteronomy the two become combined.
A question was asked some time back if the bread that Jesus broke at the Passover meal was leavened or not. According to these chapters one can not tell but according to Deuteronomy 16 it must be unleavened. The leaven represents sin and the bread represents Jesus body which was sinless.