This is one of the more obscure and most mysterious of holy days, commemorated in chapter 16 of Matthew. Or maybe Chapter 5 of Luke. The point is, I have no idea. As I understand it, Jesus is hanging out with the disciples when suddenly he says, "Hey, get a load of this!" And then a huge white fluffy cloud appears and covers everything and then it clears and Jesus is revealed to be standing in a stunning white bugle beaded robe, and Moses and Elijah are next to him, singing harmony to "This Will Be (An Everlasting Love)".
The voice of God is heard to say something very important, but no one can hear it over the music.
When the vision clears, Jesus says to the disciples, "Now you know my true nature." And the disciples fall on their face and worship him.
That's how I heard it.