Luke 9:28

And it came to pass, about an eight days after those
sayings
About a week after he had declared the above things, at, or near to Caesarea Philippi. The other evangelists, Matthew and Mark, say it was six days after: the reason of this difference is, because Luke takes in the day in which he delivered these sayings, and that in which he was transfigured, and they only reckon the intermediate days:

he took Peter, and John, and James;
the same that he admitted to be with him at the raising of Jairus's daughter, and in the garden afterwards:

and went up into a mountain to pray;
to his God and Father, that his disciples might have a visible display of his glory, as an emblem and pledge of that in which he shall hereafter appear: it was usual with Christ to go up into a mountain to pray; ( Matthew 14:23 ) ( Luke 6:12 ) . (See Gill on Matthew 17:1).