Passover was instituted in Exodus 12 when the nation of Israel was enslaved in Egypt. The Israelites were commanded by God to set aside an unblemished lamb for sacrifice and put it's blood on the door posts of their homes. The Lord spoke explaining that He would come through the land of Egypt and strike all the firstborn in the land except for those with the blood on the door post and the plague would not come over those households (Exodus 12:12-13). The lamb was a foreshadow of the sacrifice of Christ. The death of Jesus Christ takes the death penalty from us and forgive us of our sins. Through His blood we are freed of sin's death penalty (Romans 5:9; Ephesians 1:7; Colossians 1:14). The New Testament Passover explains that the unleavened bread represents Jesus Christ's broken body and the wine signifies His blood (1 Corinthians 11:27). We remember Jesus Christ's sacrifice and humble ourselves through ceremony of foot washing (John 13:8). We take part in the Passover to be purified. 

To read more about the Passover please refer to the links below.

http://www.ucg.org/tags/passover

The Passover: Why Did Jesus Christ Have to Die?

Fundamental Beliefs: The Passover