I am going to take several posts to quote William Barclay* on the transformation as I like how he explains both some of the details of this important event. Here we will look at when it happened and just which mountaintop they were on.
Mark says that this happened six days after the incidents near Caesarea Philippi. Luke says that it happened eight days afterwards. There is no discrepancy here. They both mean what we might express by saying, "About a week afterwards." Both the Eastern and Western Churches hold their remembrance of the transfiguration on 6th August...* (Do keep in mind that I like Barclay's insight into language and bygone customs, but his theology can be a bit wacky. That's not to say that I often don't find him inspiring. He can be. But just know that he should be read with caution.)
Tradition says that the transfiguration took place on the top of Mount Tabor. The Eastern Church actually calls the Festival of the Transfiguration the Taborion. It may be that the choice is based on the mention of Mount Tabor in Psalm 89:12, but it is unfortunate. Tabor is in the south of Galilee and Caesarea Philippi is away to the north. Tabor is no more than 1,000 feet high, and, in the time of Jesus, there was a fortress on the top. It is much more likely that this event too place amidst the eternal snows of Mount Hermon which is 9,200 feet high and much nearer Caesarea Philippi and where the solitude would be much more complete.
The Gospel of Mark
(The Daily Bible Series, rev. ed.)