A.T. Robertson Commentary Matthew 17:1

"And after six days Jesus taketh with him Peter, and James, and John his brother, and bringeth them up into a high mountain apart:" — Matthew 17:1 (ASV)

After six days (μεθ' ημερας εξ). This could be on the sixth day, but as Luke (Luke 9:28) puts it "about eight days" one naturally thinks of a week as the probable time, though it is not important.

Taketh with him (παραλαμβανε). Literally,

takes along . Note historical present. These three disciples form an inner group who have shown more understanding of Jesus. So at Gethsemane.

Apart (κατ' ιδιαν) means "by themselves" (alone , μονους, Mark has it) up (αναφερε) into a high mountain, probably Mount Hermon again, though we do not really know. "The Mount of Transfiguration does not concern geography" (Holtzmann).