The Nightmare Before Christmas is a stop motion film about a skeleton trying to steal Christmas. The title itself does not specify whether it is a heartwarming Christmas story or if it is a thrilling Halloween story. Technically, there is no right answer to this. No one knows for sure which it is. However, I believe that it is a Halloween movie.
Although it is not a horror movie, like the Halloween films we may be used to, it is definitely more scary than heartwarming like Christmas movies. I mean, all of the main characters are from Halloween Town. The only real Christmas character is Santa Claus, who gets kidnapped by the Halloween Town residents. If it was any other holiday that was being stolen we would consider it a Halloween movie. For example, if the Easter Bunny or a turkey was kidnapped we would not consider it an Easter or Thanksgiving movie.