From the Synaxarion:
On August 6 in the Holy Orthodox Church, we commemorate the divine Transfiguration of our Lord, God and Savior Jesus Christ.
Tabor was glorified above earth’s every region
When it looked upon God’s nature shining in glory.
On the sixth Christ transformed His form as a man.
This glorious event is recorded in the Gospels of Mark (9:2-13), Luke (9:28-36 in Orthros) and Matthew (17:1-9 in Liturgy). In the third year of His preaching, the Lord Jesus often spoke to His disciples of His approaching passion and His glorious Resurrection. So that His impending passion would not totally weaken His disciples, the All-wise Savior wanted to show them His divine glory before His passion on the Holy Cross. (This is why we sing the Katavasias of the Holy Cross on this day.) For that reason, He took Peter, James and John with Him and went out at night to Mount Tabor. The Lord took only three disciples on Tabor because the Lord did not want to leave the unworthy Judas alone at the foot of the mountain so that the betrayer would not, by that, justify his betrayal. Our Lord was transfigured on a mountain and not in a valley to teach us two virtues: love of labor and godly-thoughts. For, climbing to the heights required labor and height represents the heights of our thoughts, i.e., godly-thoughts. Moses and Elijah appeared in order to destroy the erroneous thought of the Jews that the Christ is one of the prophets; that is why He appears as a King above the prophets and that is why Moses and Elijah appear as His servants. Until then, our Lord manifested His divine power many times to the disciples but, on Mount Tabor, He manifested His divine nature. This vision of His Divinity and the hearing of the heavenly witness about Him as the Son of God would serve the disciples in the days of the Lord’s passion, in strengthening of an unwavering faith in Him and His final victory.
Unto Christ God be glory and dominion unto the ages. Amen.