A Very Sunny Christmas: Part 1
It's Always Sunny in Philadelphia Season 6 Episode 13 (12/16/10)
During the Always Sunny Christmas special, Dennis and Dee try to seek revenge against Frank for ruining Christmas for them as children. They try to prove to him he's going to die alone for being such a dick by bringing back his old business partenr, Eugene Hamilton. When that fails, they take him to his old company's Christmas party where they hide him in a couch. At the end of the episode, Frank emerges fully naked from the couch.
Meanwhile, Mac and Charlie try and relieve their old Christmas memories through a video tape and story respectively. However, both soon realize X-Mas is not what they remembered. Mac's Christmas turned out to be his father making them steal presents while Charlie's was a bunch of guys running a train on his mother.