Well, some adults still dress up for the fun of it, but they'll usually throw parties instead of going door to door. Plus you have horror movies and ghost stories... There's still ways to celebrate. It's just how you do it that changes.

And I guess it'd get stale after a while, but I don't think they care. Sora once told me that one of their residents tried to run Christmas, but it kinda blew up in his face.