How are Easter and the Jewish holiday known as Passover related?
Passover commemorates the story of the Exodus, in which the ancient Israelites were freed from slavery in Egypt. The Bible tells that God helped the Children of Israel escape slavery in Egypt by inflicting ten plagues upon the Egyptians before the Pharaoh would release his Israelite slaves; the tenth and worst of the plagues was the death of the Egyptian first-born. The Israelites were to mark the doorposts of their homes with the blood of a spring lamb and, upon seeing this, the spirit of the Lord knew to pass over the first-borns in these homes.
The crucifixion took place during the Jewish celebration of the Passover feast. Jesus was/is the perfect unblemished sacrificial lamb.