On the mountain, before Peter, James, and John, Christ is transfigured: His face shines like the sun and His garments become white as light, and Moses and Elijah appear speaking with Him. The Church understands this light not as a created glow but as the uncreated light of the Godhead, the very glory of God, which Christ possessed always and now allows His disciples to see.
The feast is the ground of a whole Orthodox understanding of salvation. What the disciples beheld on Tabor is what the saints come to share: not merely forgiveness, but transformation, the human person filled with the divine light. This is theosis, the participation in God's own life and energies, of which the Transfiguration is the promise and the foretaste.
Christ was transfigured, the hymns say, before His Passion, that when the disciples saw Him crucified they might know His suffering was freely chosen, and that the Cross leads to glory. The feast falls on August 6, in the depth of summer, a mountain of light set in the middle of the year.