itch.io

Well, most are related to the halloween easter egg.
Which entails you buying the halloween themed furniture and setting it up in your room.

For Pixie, you need to sleep a couple of nights with the furniture on.

For Gwen and Jessica, check the dungeon at night with the furniture on.

Chloe's doesn't have anything to do with the halloween event, instead you just check her room on a weekend after progressing her and Jessica.

I think those are all the ones that qualify as halloween outfits.