Christ's last meal with his apostles took place within the context of the Jewish Passover. On the first day of the Feast of Unleavened Bread, Jesus told Peter and John to go and prepare for the Passover because he desired to eat with them before he suffered. (Lk 22:8,15)
The Passover meal was not just a recalling, not merely a reenactment of the salvation the Jews experienced in Egypt, but a reliving or participation of each generation of Jews in the Covenant God had made with them and anyone who refused to participate was no longer considered as belonging to God's chosen people.
Before the meal, a lamb was sacrificed and roasted recalling that the first-born of the Jews was spared death by smearing the blood of the lamb on the door posts of each Jewish dwelling. Unleavened bread was used because the Jews had to leave Egypt in a hurry, they did not have time to bake unleavened bread. The bitter herbs eaten with the meal recalled the bitterness of Egyptian slavery. In addition, each drank four cups of wine. While drinking, each cup of wine became associated with their deliverance from Egypt as expressed in Exodus 6:6-7). The last cup was associated with Ex. 6:7 which reads: "I will take you as my own people, and you shall have me as your God." Most likely it was this fourth cup which Christ changed into his blood.
While they were eating, Jesus took bread, said the blessing, broke it,
and giving it to his disciples said,
"Take and eat; this is my body."
Then he took a cup, gave thanks, and gave it to them, saying,
"Drink from it, all of you, for this is my blood of the covenant,
which will be shed on behalf of many for the forgiveness of sins." (Mt 26:26-28)
Christ is the fulfillment of the Jewish Passover as we read in the Catechism of the Catholic Church.
By celebrating the Last Supper with his apostles in the course of the Passover
Jesus gave the Jewish Passover its definitive meaning.
Jesus' passing over to his father by his death and Resurrection, the new Passover,
is anticipated in the Supper and celebrated in the Eucharist,
which fulfills the Jewish Passover and anticipates the final Passover
of the Church in the glory of the kingdom."
Many attempts have been made during the 2,000 years of Christian history to explain "This is my body...this is my blood" symbolically or spiritually but all have failed to do justice to the reality. The Semitic expressions "body" and "blood" represent the whole human being. Although Christ had not as yet shed his blood, he had already sacrificed in intention his life to the Father. "By the divine power of his determinative words Jesus changes bread and wine into his sacrificial person." (Sacramentum Mundi, p. 451) It now became clearer to the apostles as to how Christ would give them his body and blood to eat but did not do away with the seeming inherent contradiction.
A discussion arose in the Middle Ages as to the kind of body Christ gave to the apostles at the Last Supper. Some thought it was his impassible or immortal body. But, how could this be since Christ had not a yet died and risen? St. Thomas Aquinas maintained that Christ gave the apostles his mortal body to eat but in a sacramental manner, that is, by becoming really present in his sacrificial humanity under the appearances of bread and wine. (Summa, Pat III, Q. 81, Art. 3)