After six days Jesus took with Him Peter, James, and John the brother of James, and led them up a high mountain by themselves.
Matthew 17:1. And after six days. We must first inquire for what purpose Christ clothed himself with heavenly glory for a short time, and why he did not admit more than three of his disciples to be spectators. Some think that he did so, in order to fortify them against the trial which they were soon to meet with, arising from his death.
We have here the story of Christ's transfiguration; he had said that the Son of man should shortly come in his kingdom, with which promise all the three evangelists industriously connect this story; as if Christ's transfiguration were intended for a specimen and an earnest of the kingdom of Christ, and of that light and love of his, which therein appears to his select and sanctified ones.
Commenting on Matthew 17:1-13
MATTHEW CHAPTER 17 Verses 1-9. See also Verse 1. And after six days. That is, six days from the conversation recorded in the last chapter. Luke says, about eight days after. Matthew mentions the six days that intervened between the day of the conversation and the transfiguration. Luke includes both those days, and thus reckons eight. Besides, Luke does not pretend to fix the precise time.