I could probably fill just about this entire month with horror movies set at Christmastime; for some reason, it's an extremely popular holiday for genre filmmakers, and might even edge out Halloween as the top horror holiday. The contrast between the joy and togetherness of Christmas and the terror of murder and mayhem is apparently just too hard to resist. I don't know if Black Christmas is the first yuletide horror film, but it's certainly the most well-known, a cult classic that's often credited with setting the template for the slasher films that reigned in the 1980s (it was also remade to generally poor notices in 2006).
You can see the seeds of the slasher genre here, as a mysterious killer goes one by one through the members of a sorority who have stayed behind on their college campus during the holiday break. Director Bob Clark (who went on to make a very different Christmas movie, A Christmas Story, about a decade later) builds things very slowly, though, and he holds back much of the gore that would soon become a hallmark of the genre. He spends as much time on character development, even of the characters who get killed early on, as he does on suspense, and gives surprising weight to a subplot about Olivia Hussey's sorority sister considering an abortion.
Hussey, Margot Kidder and Andrea Martin are all pretty solid as the main sorority girls, and keep things interesting when the killer fades into the background. Parts of the movie definitely drag, and some of the dialogue is clunky, but Clark does a good job of sustaining the creepiness, thanks to the eerie phone calls that the killer periodically makes to the sorority. Clark uses the holiday for moments of comic relief (an obnoxious frat boy playing Santa freely swears in front of small children) and disturbing juxtaposition (a great scene alternates children caroling with a character getting stabbed, giving Christmas music the unnerving context it deserves), but never for cutesy irony or heavy-handed messages. And all the slasher conventions kind of fade away with the ambiguous ending, which resolves nothing and leaves things far more unsettled than one would expect from a horror movie about sorority girls. I literally watched through the credits waiting for some kind of explanation, but it never comes. Evil is still out there, unvanquished, unexplained, waiting for next Christmas.
The True Meaning of Christmas: Make sure to go home for the holidays, otherwise you may get brutally murdered.