Welcome to Passover, the Feast of the Unleavend Bread. For those who don't know, the feast is in commemoration of the Hebrew exodus from Egypt (important parts for feast: Exodus 11:1-12:50).
It was the celebration of this feast that brought Christ to Jerusalem. Christ sends his disciples out to find a room for the meal, and they do. They set up the traditional meal and do what is customary. But then things start to turn weird.
During the dinner, Christ makes an announcement. He is going to be betrayed. Keep in mind that at this point, he has already predicted his death. But betrayal? And furthermore, he tells the disciples that it will be one of them who will do the deed.
After this, they start eating again, but Jesus, being the Teacher that he is, descides to do something unusual with the meal. He took the bread (unleavened), broke it, and gave it to the disciples saying it was his body. Likewise, he took the cup, gave thanks, and told them to drink, saying it was his blood. These sayings have caused a lot of controversy over whether or not Communion is literally the Body and Blood of Christ or whether he is merely present in spirit. But that's not the point of Communion. The point is that Communion serves as the sign for Christ's new covenant. The old covenant declared the Jewish people the chosen people of God. The new covenant declares freedom for the captives, peace, and forgiveness.