Albert Barnes on Matthew 17:1

And after six days. That is, six days from the conversation recorded in the last chapter. Luke Lk 9:28 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. In the Greek it is "about eight days after."

Taketh Peter, James, and John. These three disciples were with him also in the garden of Gethsemane, Mk 14:33. He designed to fit them, in an eminent degree, for the work of the gospel ministry, by the previous manifestations of his glory, and of his patience in suffering.

Into an high mountain apart. That is, apart from the other disciples. It is commonly supposed that this was Mount Tabor, a high mountain in Galilee. The name of the mountain is not, however, mentioned in the New Testament. Luke adds, Lk 9:28 that he ascended there to pray. Our Saviour prayed much. When he did it, he chose to be alone. For this purpose he often ascended mountains, or went into the deserts. There is something in the solitude and deep and awful stillness of a lofty mountain favourable to devotion.

(g) "And after" Mk 9:2, Lk 9:28