When you think about it, isn't the Transfiguration kind of an odd choice for a feast?
I mean, sure, it's a dramatic story, an inexhaustible font of fruitful meditation, good homilies, and fine art.
But in terms of advancing the story of Jesus through the liturgical year, what really happened on Mount Tabor? The witnesses to this awesome theophany first babbled, then forgot about it, then reverted to the same squabbling bumblers they had always been.
The Transfiguration is a moment of profound theology plopped like a blinking neon light into a sequence of stories of Jesus' teaching and healing. It seems like the sort of thing that belongs in John's Gospel, rather than the synoptics.
Why the Feast of the Transfiguration, though, and not the Feast of the Multiplication of the Loaves and Fishes, or the Feast of the Wedding at Cana?
The old Catholic Encyclopedia suggests it was instituted as a practical substitution for a pagan feast. The role it seems to play in the current Roman Catholic calendar, though, seems to be the same it plays in the Gospels: a sudden confrontation with the declaration of God:
This is My chosen Son; listen to Him.
The implications of this are, as I say, inexhaustible, but I think that, fundamentally, the Feast of the Transfiguration is about a loud, startling noise or a splash of cold water to shake us out of the kircheschlaft routine of religion and say, "Hey, you know what? All that stuff we say about Jesus? IT'S TRUE!"