We do not know exactly on which mountain Jesus was when the transfiguration took place. The place, however, was holy because it was where God revealed his presence in a special way. Though Mount Zion and the Mount of Transfiguration are never identified in Scripture as being the same, Peter creates a link by referring to the holy mountain. He wants us to understand that Jesus is the King of Psalm 2:1–12.1
18 we ourselves heard this very voice borne from heaven, for we were with him on the holy mountain.