Type of Feast:
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.
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).
Despite the importance of this event, the Feast of the Transfiguration was not among the earliest of the Christian feasts. It was celebrated in Asia starting in the fourth or fifth century and spread throughout the Christian East in the centuries following. The Catholic Encyclopedia notes that it wasn't commonly celebrated in the West until the tenth century. To celebrate the great Christian victory at the Siege of Belgrade in 1456, during which the Muslim Turks were routed and the Islamic advance into Europe was halted, Pope Callixtus III elevated the Transfiguration to a feast of the universal Church and established August 6 as the date of its celebration.