“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.”
And Peter said to Jesus: Lord, it is good that we are here. If you wish, I will make three tents here, one for you and one for Moses and one for Elijah. Peter's response to the transfiguration — building three tents — reflects the Festival of Tabernacles practice of dwelling in booths, or possibly the desire to prolong and institutionalize the experience. Mark 9:6 notes that Peter did not know what to say. The three-tent proposal treats the glorified Christ as one equal among three rather than as the one who fulfills what the other two represent.
Church Fathers on Matthew 17:4
In this Transfiguration undergone on the mount, the Lord fulfilled within six days the promise made to His disciples, that they should have a sight of His glory; as it is said, 'And after six days he took Peter, and James, and John his brother.'
It is made a question how it could be after six days that He took them, when Luke says eight. The answer is easy, that here one reckoned only the intervening days, there the first and the last are also added.
He does not take them up immediately upon the promise being made, but six days after, for this reason, that the other disciples might not be touched with any human passion, as a feeling of jealousy; or else that during these days' space, those disciples who were to be taken up might become kindled with a more eager desire.