They traveled 6 days before the transfiguration... may be a nod to Exodus 24:15... Moses spent 6 days in the glory cloud. Also, Adam was made on the 6th day, and now the new Adam will defeat the serpent the first Adam failed to defeat. Luke says 8 days, not 6. He delineates the time differently.

The mountain... which one? Maybe Mt. Tabor. Maybe Mt. Hermon at Caesarea Phillippi... if so, it may be the site where the "watchers" described in the non-canonical book of 1 Enoch and mentioned in Genesis 6 spread their false teaching. That said, Scripture doesn't mention the name of the mountain, so we can consider it non-essential, doctrinally.

Moses and Elijah were both forerunners, so their appearance here seems to be preparing the way for Jesus as Messiah.

Peter is terrified, doesn't know what to say and starts filling the space with crazy talk, recommending they build tents for Jesus, Moses and Elijah to stay there on the mountain. He misses that Jesus is not just on par with Moses and Elijah, but is actually God himself.

The cloud in verse 7 is the glory cloud, the Spirit of God from the temple in the Old Testament. So we now have the full trinity: the Father, Son and Holy Spirit, all present on the mountain. They are all involved in the ministry of Jesus!

The glory, the splendor of God that we see here is apparently the the glory of God the Father, as God the Son is depicted bathed in the light (and God the Spirit is a cloud overshadowing them). When we pass from this world to the next, we too will be bathed in the light and glory of the Father!

Our God is very quick to give honor. By contrast, when we're walking in the flesh, we're very slow to give honor to anything but ourselves. This can cause trouble in all sorts of contexts, including marriage. God gives us honor, a good reputation, calls us to serve in the church, lets us work for his Kingdom. He's lavish with the glory he bestows on us! And we should be just as generous with others as he's been with us. We should be quick to build up other people, to serve other people; we lose nothing when we do!

Despite his supreme worthiness of glory and honor, and despite what we deserve in our sin nature, God is generous in giving glory and honor to us. We will one day be glorified as he is glorified! He laid down his life for us, and we ought to lay down ours for others as well. He gives to us so that we give to others! In this way, we become glorified, more like him.