And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart, After six days - Mark 9:2, has the same number; but Luke says, Luke 9:28, after eight days. The reason of this difference seems to be the following: Matthew and Mark reckon the days from that mentioned in the preceding chapter, to that mentioned in this; Luke includes both days, as well as the six intermediate: hence, the one makes eight, the other six, without any contradiction.
Peter, James, and John - He chose those that they might be witnesses of his transfiguration: two or three witnesses being required by the Scripture to substantiate any fact. Eminent communications of the Divine favor prepare for, and entitle to, great services and great conflicts. The same three were made witnesses of his agony in the garden, Matthew 26:37.
A high mountain - This was one of the mountains of Galilee; but whether Mount Tabor or not, is uncertain. Some think it was Mount Hermon. St. Luke says, Christ and his disciples went up into the mountain to pray, Luke 9:28.