The Transfiguration of Our Lord Jesus Christ

Collect for the Feast of the Transfiguration (from the Mass of St. Pius V)

O God, Who in the glorious transfiguration of Thine only-begotten Son didst strengthen the sacraments of faith by the testimony of the fathers, and Who didst  wonderfully foreshow the perfect adoption of Thy children by a voice coming down in a shining cloud, mercifully grant that we be made co-heirs of the King of glory Himself, and grant us to be sharers in that very glory. Through the same Lord Jesus Christ, Thy Son, Who liveth and reigneth with Thee in the unity of the Holy Ghost, God, world without end. Amen.

History:

The feast of the Transfiguration of Christ celebrates the revelation of Christ's divine glory on Mount Tabor in Galilee (Matthew 17:1-6; Mark 9:1-8; Luke 9:28-36). After revealing to His disciples that He would be put to death in Jerusalem (Matthew 16:21), Christ, along with Ss. Peter, James, and John, went up the mountain. There, St. Matthew writes, "he was transfigured before them. And his face did shine as the sun: and his garments became white as snow." The brightness was not something added to Christ but the manifestation of His true divine nature. For Peter, James, and John, it was also a glimpse of the glories of heaven and of the resurrected body promised to all Christians. As Christ was transfigured, two others appeared with Him: Moses, representing the Old Testament Law, and Elijah, representing the prophets. Thus Christ, Who stood between the two and spoke with them, appeared to the disciples as the fulfillment of both the Law and the prophets.
At Christ's baptism in the Jordan, the voice of God the Father was heard to proclaim that "This is my beloved Son" (Matthew 3:17). During the Transfiguration, God the Father pronounced the same words (Matthew 17:5).