it's simple really: october is for horror movies with highly stylized villains and iconic monsters because they make for good costumes. early november is for gothic horror because of the fog and decay etc. late november is for zombies (commentary on consumerist culture). december is for psychological horror and stuff where they're trapped in a room because that's when you have to go to holiday parties with your family.
january is for possession and body horror (the loss of control and transformation of a new year beginning plus toxic diet culture). february is valentine's day so you can do whatever as long as you're horny about it.
march is for religious/cult-themed horror because of lent and easter. obviously april is giallo because the blood splatter represents spring rainfall. needs no explanation
may is for cannibalism and horror comedy (taurus season)
all of summer is for urban legends and anything that features children with minimal adult supervision so kids can watch them and be scarred for life. september is slashers because it's back to school season and that shit always seems to happen to teenagers and college students