What is the ruling on watching horror films on Halloween, with the intention of taking advantage of the "excuse" of watching "horror" films on the "horror" day?

It is not permissible to celebrate the aforementioned holiday because it is one of the holidays of the polytheists. A Muslim should not do on the polytheists' holiday what he does not do on other days, in order to avoid outward imitation. He is not sinful by his action unless he intends to imitate them.