A Princess Carol is a My Little Pony: Friendship is Magic fanfic by John Perry. The plot is pretty simple: Princess Luna just doesn't understand how the quiet quaint holiday she knew has become the rowdy crass holiday known as Hearth's Warming Eve. Her sister decides the best way to show her the error of her judgement is by using the plot of the popular story. (Yes, even in Equestria A Christmas Carol, or some version of it, is played a billion times every December.) Celestia herself is the Ghost of Hearth's Warming Past, Fluttershy is Present, and Zecora is the Future.
This fic is very funny, a scene near the end nearly made me cry, and it managed to be as heartwarming as the original story. I also love the moral, about how we shouldn't try to force ponies to celebrate holidays how they originally were, everypony should just celebrate it the way they want to. Very well done, and not too long, fic, I highly recommend it to all bronies and fans of the original Christmas Carol. Find it here. Laters.