The Tabernacles Vision
The "transfiguration" event was a visual Prophecy of the yet-future coming of the Kingdom of God to earth (see Feast Of Tabernacles: The Gospel Of The Kingdom). The apostles Peter, James and John were given to witness The Messiah with Moses and Elijah (rendered by the King James Version as "Elias" in the New Testament) in the time after Christ's return when the first resurrection (see When Will You Be Judged?) of all of the LORD's people through human existence will be resurrected - including Moses and Elijah who are now dead (see What Happens When You Die?; also Why Did Christ Put Moses To Death? and Where Did Elijah Go?).
"17:1 And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart, 17:2 And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light. 17:3 And, behold, there appeared unto them Moses and Elias talking with him." (Matthew 17:1-3 KJV)
"Let us make here three tabernacles; one for Thee, and one for Moses, and one for Elias"
The Christian Feast of Tabernacles (see Feast Of Tabernacles: The Gospel Of The Kingdom) is about the time portrayed in the "transfiguration." Whether or not Peter was fully aware of what he was saying, his suggestion to build tabernacles was perfectly appropriate to what was happening in the vision.
"17:4 Then answered Peter, and said unto Jesus, Lord, it is good for us to be here: if thou wilt, let us make here three tabernacles; one for thee, and one for Moses, and one for Elias." (Matthew 17:4 KJV)
The coming of the Kingdom of God means exactly what it says. The Son of God has been sent by The Father to prepare the way for The Father's coming (see The Throne Of God, From Heaven To Earth).
"17:5 While he yet spake, behold, a bright cloud [see also The Clouds of Heaven] overshadowed them: and behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye him.
Jesus (in the actual Hebrew pronounced Yeshua) then explained how and why, since the original Elijah was dead, another prophet, John the Baptist, was The Elijah To Come for that time.
"17:9 And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.
Fact Finder: How do all of the true Christian Holy Days point to the coming Kingdom of God?