In Exodus 11 the Lord announced the tenth plague, which would be the death of the firstborn. Then in chapter 12 He instituted the Passover. Each family took a lamb and sacrificed it to God. They put some of the blood on the doorposts and lintel of the houses. They then ate the lamb along with bitter herbs and unleavened bread.
Exodus 12:13 "Now the blood shall be a sign for you 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 strike the land of Egypt."
So Israel kept the Passover every year as a reminder of their deliverance from the bondage of Egypt.
In 1 Cor. 5:7 we have the comparison of the death of Jesus to the death of the Passover lamb.
"Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us."
Our Passover lamb is not one from a flock. Paul says Jesus Christ is our Passover lamb and our lamb has already been sacrificed. During the very season when Israel offered its Passover lambs, the true Lamb of God was offered for us. We can see why John the Baptist said of Christ in John 1:29, "Behold! The Lamb of God who takes away the sin of the world!" He is the means by which we are delivered from the bondage of sin.
We eat the Lord's Supper in remembrance of Him and the deliverance we have from bondage through the blood of the Lamb.