Corpus Christi (Latin for Body of Christ) is a Christian feast. Its purpose is to honor the Eucharist, and as such it does not commemorate a particular event in Jesus' life. Its celebration on the Thursday following Trinity Sunday is meant to associate it with Jesus' institution of the Eucharist during the Last Supper, on Maundy Thursday. Because of the sorrow of Holy Week, no festivals are celebrated within it; the Thursday after Trinity Sunday is the first Thursday after Holy Week, Eastertide, and the (now obsolete in the ordinary form) Octave of Pentecost have ended.
Propers for the Feast of Corpus Christi
O GOD, who in a wonderful Sacrament hast left unto us a memorial of thy Passion: grant us, we beseech thee, so to venerate the sacred mysteries of thy Body and Blood, that we may ever perceive within ourselves the fruit of thy redemption. Who with God the Father livest and reignest with thee in the unity of the Holy Ghost, ever one God, world without end. Amen.
The Epistle - 1 Corinthians xi. 23.
I have received of the Lord that which also I delivered unto you, That the Lord Jesus the same night in which he was betrayed took bread: and when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me. After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me. For as often as ye eat this bread, and drink this cup, ye do shew the Lord's death till he come. Wherefore whosoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord. But let a man examine himself, and so let him eat of that bread, and drink of that cup. For he that eateth and drinketh unworthily, eateth and drinketh damnation to himself, not discerning the Lord's body.
The Gospel - St. John vi. 55.
Jesus said to the multitudes of the Jews: My flesh is meat indeed, and my blood is drink indeed. He that eateth my flesh, and drinketh my blood, dwelleth in me, and I in him. As the living Father hath sent me, and I live by the Father, so he that eateth me, even he shall live by me. This is that bread which came down from heaven; not as your fathers did eat manna and are dead; he that eateth of this bread shall live for ever.