It's a Wonderful Life to Die Hard to The Holdovers. The films you put on once a year and the ones whose Christmas setting is structurally argued.
The Christmas film is a category whose membership has been contested. Films explicitly about Christmas (It's a Wonderful Life, A Christmas Story) belong to the category uncontroversially. Films that happen to be set during the Christmas season (Die Hard, The Apartment, Eyes Wide Shut, Carol) belong to the category by argument — defenders argue that the setting is structurally important to the films; critics argue that the films are not, in any meaningful sense, Christmas films at all.
We've included both categories. Our picks across nine decades.
Die Hard's defenders argue that the Christmas setting is the structural foundation of the film's plot mechanics. The office Christmas party is what brings the characters together; the holiday's family-and-reunion theme is what motivates John McClane's mission; the specific seasonal vocabulary (Ho-ho-ho on the dead terrorist's sweatshirt, Christmas in Hollis on the soundtrack) is integrated throughout. By this reading, Die Hard is structurally as much a Christmas film as it is an action film.
The same argument applies to Eyes Wide Shut, where Kubrick's deliberate Christmas-light decoration of almost every interior produces the film's specific visual register, and to Carol, where the Christmas-shopping setting is the engine of the central encounter. The 'is it a Christmas film' debate is, in some sense, a debate about what the genre's definition actually is. The films above either earn membership in the category through their content (the canonical entries) or through the structural function of their seasonal setting (the contested entries).