What do these two have to do with each other: one, a battle fought in 1456 against Moslem invaders and the other, the mystery of Our Lord’s manifesting His glory to three of His Apostles on Mount Tabor? On July 22, 1456, the Christian forces under the command of the great Hungarian Janos (John) Hunyadi roundly defeated the Turkish Moslem forces of Sultan Mehmet II. On the sixth of August, the news reached Rome. In gratitude, Pope Callistus III elevated the Feast of the Transfiguration to the universal calendar.
Instituting new liturgical feasts on the occasion of deliverance from an enemy is a custom going back to the Old Testament. The feast of Purim celebrates the deliverance of the Jews in the time of Queen Esther. Chanukah (The Festival of Lights) commemorates the rebuilding of the Temple after the defeat of the Syrians by the Maccabees. Both of these festivals were legitimate Jewish practices at the time of Our Lord.
New Testament feasts instituted or elevated in importance because of military victories include the Feast of the Holy Rosary, October 7 (the battle of Lepanto), and the Feast of the Holy Name of Mary, September 12 (the battle of Vienna). Callistus III certainly did not institute a new feast in the Transfiguration, but elevated an ancient one to a higher status. From antiquity, the feast already existed in the East. In the Arab Christian world it is known as the “feast of the Lord.” The Armenian Church names one of the principal divisions of its liturgical year after it. In the West, the Feast of the Transfiguration goes back at least to the ninth century, but was not so universally kept, August sixth being more commonly used to commemorate St. Sixtus II.