Black Christmas (1974)
Directed by Bob Clark
I thought it would be best to begin the 13 Nights of X-Mas with the film that arguably started the slasher subgenre: Black Christmas. It spawned countless imitators, including Halloween and Friday the 13th, and led to every holiday-themed horror film from My Bloody Valentine to Leprechaun. It established a number of elements that would later become horror clichés. Black Christmas is not just a landmark film within the horror genre, but an important piece of film history in general.
