The Jewish Festival of Booths was a feast of the dwelling of God with men, and the transfiguration of Christ reveals how this dwelling takes place in and through the Messiah, the Son of God in human flesh. There is little doubt that Christ’s transfiguration took place at the time of the Festival of Booths, and that the celebration of the event in the Christian Church became the New Testamental fulfillment of the Old Testamental feast in a way similar to the feasts of Passover and Pentecost
In the Transfiguration, the apostles see the glory of the Kingdom of God present in majesty in the person of Christ. They see that in him, indeed, “all the fullness of God was pleased to dwell,” that “in him the whole fullness of deity dwells bodily” [Col. 1:19, 2:9]. They see this before the crucifixion so that in the resurrection they might know who it is who has suffered for them, and what it is that this one, who is God, has prepared for those who love him. This is what the Church celebrates in the feast of the Transfiguration.
"On this day, O Lord, You manifested Yourself to the disciples on Mount Tabor. And Your disciples being afraid, cried aloud saying, Let us make three tabernacles, one for You, Lord, and one for Moses and one for Elijah. And now, we also, Your acceptable flock, sing together with Your true witnesses. Send down upon us the light of Your grace at Your second coming and save us.[Introit]
Besides the fundamental meaning which the event of the Transfiguration has in the context of the life and mission of Christ, and in addition to the theme of the glory of God which is revealed in all of its divine splendor in the face of the Saviour, the presence of Moses and Elijah is also of great significance for the understanding and celebration of the feast. Moses and Elijah, according to the liturgical verses, are not only the greatest figures of the Old Testament who now come to worship the Son of God in glory, they also are not merely two of the holy men to whom God has revealed himself in the prefigurative theophanies of the Old Covenant of Israel. These two figures actually stand for the Old Testament itself: Moses for the Law and Elijah for the Prophets. And Christ is the fulfillment of the Law and the Prophets [Mt 5:17].
They also stand for the living and dead, for Moses died and his burial place is known, while Elijah was taken alive into heaven in order to appear again to announce the time of God's salvation in Christ the Messiah. Thus, in appearing with Jesus on the mount of Transfiguration, Moses and Elijah show that the Messiah Saviour is here, and that he is the Son of God to whom the Father himself bears witness, the Lord of all creation, of the Old and New Testaments, of the living and the dead. The Transfiguration of Christ in itself is the fulfillment of all of the theophanies and manifestations of God, a fulfillment made perfect and complete in the person of Christ. The Transfiguration of Christ reveals to us our ultimate destiny as Christians, the ultimate destiny of all men and all creation to be transformed and glorified by the majestic splendor of God himself. The Gospel reading of the day is from St. Matthew 16:13-17:13.
Excerpt from: Samoorian, V. Rev. Fr. Ghevont. DOMAR: A Compendium of Directorium and Calendar of the Armenian Apostolic Orthodox Church according to the traditions of the Apostolic See of Jerusalem. Armenian Orthodox Theological Research Institute [AOTRI], 2006.