I heard that the Christmas episode has got something to do with an Anti-Santa and/or a couple who think they're Pagan gods.
There is also a flashback of young Sammy. // Copied and pasted from TV.com's Supernatural page:
In this Christmas-themed episode, Sam and Dean encounter a twisted couple that insist they are Pagan gods and share a craving for human meat. Flashbacks follow the Winchesters during Christmas 16 years ago when Sam first learned that demons existed and that his father hunted them. // And this is from The Futon Critic:
SAM AND DEAN HUNT AN ANTI-SANTA It's Christmas time and Sam (Jared Padalecki) and Dean (Jensen Ackles) investigate a series of murders where the victims were pulled up through the chimney. Sam realizes they are dealing with a sort of Anti-Santa, a demon with roots in pagan lore. Dean wants to celebrate Christmas the old fashioned way as this is his last, but Sam refuses, not wanting to accept that Dean won't be around next year. Sam flashes back to a certain Christmas when he waited for his father to come home to give him a special gift. J. Miller Tobin directed the episode written by Jeremy Carver (#308).