Surprisingly, widespread commemoration of the Feast of the Transfiguration is of relatively recent vintage in the West.
August 6 marks the Feast of the Transfiguration of Christ, a major event in Our Lord’s earthly ministry where, for the second time, God the Father proclaimed Jesus to be His only-begotten and beloved Son. By shining forth to Peter, John, and James on Mount Tabor, Christ revealed His Divine Glory to the Disciples and strengthened their faith before the Passion.
This is explained in St. John Chrysostom’s homily on the Gospel of St. Matthew, which comprises the first reading from Matins in the 1962 edition of the Breviarium Romanum.
Since the Lord had spoken much concerning dangers, much concerning His Own sufferings, much concerning death, and the killing of His disciples, and had laid upon them many hard and grievous things, and since all these were in this present life, and already hanging over them, whereas the good things were matter for hope and waiting as, for example, that whosoever should lose his life for His sake should find it, for that the Son of Man should come in the glory of His Father, and reward every man according to his works. (Matth. xvi. 25, 27). Therefore, to assure them by their own eyes, and show them what the glory is wherein He will come, He manifested and unveiled it to them, as far as in this life they were able to grasp it, lest they and especially Peter should grieve over their own deaths, or the death of their Lord.
Surprisingly, widespread commemoration of the Feast of the Transfiguration is of relatively recent vintage in the West. The earliest record of its celebration among the Latins is the 9th Century, and for many centuries it was only kept as a local feast on differing days. Then, in 1456, Pope Calixtus III extended the feast to the entire Church due to the Hugarian victory over the Turks in Belgrade on August 6 of that year.
Among Eastern Christians, the Transfiguration is considered one of the great feasts of the liturgical year. In the Byzantine Rite (which is observed by Greek Catholics), the Transfiguration is ranked as a Vigil (the highest classification) and kept with an Octave. More specifically, the rubrics call that on the evening before the feast, Vespers, Matins (which, in the Byzantine Rite, includes Lauds), and First Hour (Prime) are to be celebrated as one service with great solemnity. In times past, this liturgical commemoration would proceed throughout the evening and into the early hours of the next morning, hence why it is classified a Vigil. Today, however, this practice is rarely followed outside of monastic settings, and is mostly kept among the Eastern Slavs.
The Feast of the Transfiguration is considered so important in the Byzantine Rite that it marks an exception to the Dormition (Assumption) Fast that runs through the first 14 days of August. According to traditional Eastern fasting rules, the Transfiguration marked one of the few days during this period where fish could be served; otherwise the faithful were expected to forego fish, along with meat, dairy, and oil until Our Lady's Feast on August 15.
In order to provide a sample of how the Byzantine Rite keeps the Feast of the Transfiguration, we close with the text of two brief hymns that are recited during the liturgy on August 6. Both capture the essence of the celebration while giving glory to God for this great revelation.
Thou wast transfigured on the mountain, O Christ God, Who didst show Thy glory unto Thy disciples as far as they could bear it. May Thine ever-existing light shine forth also upon us sinners. Through the prayers of the Mother of God, O Bestower of light, glory be to Thee!
On the mountain wast Thou transfigured, and Thy disciples beheld Thy glory as far as they could bear it, O Christ God; that when they would see Thee crucified they would comprehend that Thy suffering was voluntary, and proclaim to the world that Thou art of a truth the Effulgence of the Father.