The Feast of the Transfiguration
Our Lord, in order to show that the sufferings of His servants are usually intermingled with the spiritual comforts, and to give us a sensible demonstration of the truth of His promises of an eternal glory reserved for us in the world to come, was pleased to manifest His majesty in the mystery of the Transfiguration.
Five days after the great confession of St. Peter at Caesarea Philippi and the consequent prediction of Messiah's coming death and resurrection, a vision is granted to the three Apostles who were taken into Our Lord's closest intimacy, which carries them to a further stage of knowledge of His true nature. Our Lord chose to be witnesses of His glory the same three beloved disciples who were afterward to be witnesses of His agony in the garden, namely Peter, James, and John. He took three, that their evidence might be unexceptional; but he would not publicly show His glory, for the closest secrecy is consistent with all spiritual graces. Jesus therefore showed this mystery in retirement and He led these three apostles to a lonely mountain, as He was accustomed to go often to some solitude to pray.
He whom Peter had recognized at Caesarea Philippi to be Messiah, who had just told them that Messiah would come "in the glory of His Father," He, they learn, was not only Son of Man but Son of God. The Father, who had borne that witness to Jesus Himself at the Baptism now bears the same witness to the three Apostles.
More and more the work of the Lord is done on a smaller and smaller circle. If the foundations were to carry the great building of redeemed humanity, they must be dug deep and out of sight. Not only does the public ministry of Our Lord contract itself, but even what Our Lord did "privately" meant first with his own disciples, i.e. followers; then with the Twelve Apostles; and now in the Transfiguration and later in Gethsemane with only the Three of the Twelve.
This mystery was significant as showing the testimony of the Jewish Law and Prophets to the Messiahship of Christ (the appearance of Moses, Elijah with Our Lord); as showing a further Divine proclamation of Our Lord's Sonship ("this is my beloved Son") ; and as foreshadowing the future glory of the Son crucified.
The tradition of the Christians in Palestine, of which the Fathers speak, assures us that this mountain was Mount Tabor, which is very near to Nazareth. However, nearly all scholars today agree that is is rather Mount Hermon (9000 feet) between Caesarea Philippi and Damascus.
This feast originated in the Eastern Church, where it had become widely celebrated before 1000. In the West, its general observance goes back to 1457, when Gallistus III ordered its universal celebration in commemoration of the victory gained over the Turks at Belgrade on August 6, 1456.
—From Notes on Movable and Immovable Holy Days (taken from Butler's Lives of the Saints; Cross's Oxford Dictionary of the Christian Church; Catholic Encyclopedia; and Shepherd's Oxford American Prayer Book Commentary).