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 Exodus 12:12-13 [12] For I will pass through the land of Egypt this night, and will smite all the firstborn in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgment: I am the LORD. [13] And the blood shall be to you for a token on the houses where you are: and when I see the blood, I will pass over you, and the plague shall not be on you to destroy you, when I smite the land of Egypt.
American King James Version×). 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 Romans 5:9Much more then, being now justified by his blood, we shall be saved from wrath through him.
American King James Version×; Ephesians 1:7 Ephesians 1:7In whom we have redemption through his blood, the forgiveness of sins, according to the riches of his grace;
American King James Version×; Colossians 1:14 Colossians 1:14 In whom we have redemption through his blood, even the forgiveness of sins:
American King James Version×). 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 1 Corinthians 11:27Why whoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord.
American King James Version×). We remember Jesus Christ's sacrifice and humble ourselves through ceremony of foot washing (John 13:8 John 13:8Peter said to him, You shall never wash my feet. Jesus answered him, If I wash you not, you have no part with me.
American King James Version×). 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