Some of my favorite atmospheric movies for Halloween that have a seasonal ambiance:
Curse of The Demon--an old black and white film. It is well made and not cheesy. Takes place in the UK with Halloween briefly in the background. An American professor arrives in London for a parapsychology conference, only to find himself investigating the mysterious actions of Devil-worshiper and his small cult. The silly monster illustrated was added later in brief bits because someone felt that the movie needed a monster.
https://www.youtube.com/watch?v=SRiS7De2hs4
One that scared the crap out of me as a kid: City of The Dead. A B&W film with strong seasonal ambiance. A young college student arrives in a sleepy Massachusetts town to research witchcraft; during her stay at an eerie inn, she discovers a startling secret about the town and its inhabitants. the trailer and a link to the whole film on Youtube.
First the trailer:
https://www.youtube.com/watch?v=8fAJAHfQB6Q
Now the entire movie:
https://www.youtube.com/watch?v=vF7IB3n3UbU