I like to think that each Hogwarts house has its own Halloween party after the big feast, but each of them very different from each other.
I feel like the Gryffindors would have a pretty standard party with spooky decorations, snacks, and drinks. But you won’t see any of the costumes that are common in the muggle world, like bedsheet ghosts or witches with brooms that would never get off the ground. There’s nothing scary about that for a wizard, and in fact, they find it a bit insulting.
Hufflepuffs, on the other hand, have fully embraced the muggle tradition of wearing costumes. They spend weeks planning group costumes with their friends. You might see a quidditch team, a group of muggle superheroes, a set of potions ingredients, the four seasons, or a little flock of owls. One memorable Halloween, a group even came as the Hogwarts Express, costumes all linked together, enchanted smoke billowing out of the engine.
Ravenclaws sit around the fire in the common room telling scary stories. Some of them make up stories about ghouls, lethifolds, and dementors, while others adapt muggle horror movies or disturbing stories from the Daily Prophet. They’ve also developed a tradition of “riddle costumes” – that is, costumes whose meaning is not immediately obvious. The harder time your friends have figuring out what you’re dressed as, the better job you’ve done – as long as the answer makes sense once you’ve explained.
Slytherins like to see themselves as a bit “above it all”. After all, Halloween is just a pale muggle imitation of a world they live in every day. But even they can’t resist a chance to sip pumpkin juice (or something stronger) and munch on chocolate. Every once in a while, a newcomer like Albus Potter will try to decorate the common room with fake cobwebs and plastic spiders, but it never catches on.