The God in Red is a christmas themed short-story about an apprentice theurgist messing up when trying to summon the goddess Piskor the Generous and getting Santa Claus instead. As the theurgists try to get the spirit of Christmas home, and everyone tries to figure out what's going on, hilarity ensues.
Lawrence Watt-Evans has posted this short story online; http://www.ethshar.com/thegodinred0.html