why is the nightmare before christmas a halloween movie

Well, it's a bit of a long story. The Nightmare Before Christmas was actually released in theaters in October of 1993, which is why it's considered a "Halloween movie." It was released as part of Disney's annual "Halloween Treat" program, which featured other spooky-themed movies like "The Adventures of Ichabod and Mr. Toad" and "Cinderella." However, it's still a Christmas movie at its core, so it's often shown during the holiday season as well.