The mane six have been imbued with the power of the Rainbow of Harmony. Celestia and Luna have come to them, informing them that their presence is requested: they need to make a pilgrimage to the monastery of the Order of Harmony, to be taught the deeper secrets of their new magic.
One catch: they must go as supplicants. Which means that they all have to shave off their manes and tails.
Rarity is not pleased, and she proceeds to take great pains to let them all know WHY.
An alternative take on the story Last Evening Together by Pen Stroke...