The Halloween costumes for Jessica and Gwen can be unlocked by getting the dungeon for the manor and going there at night iirc, and there should be an event that plays and after that event you can replay it and the skins get unlocked. Hopefully this helps!
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Adding to this.
Jessica and Gwen's event for the costumes only happen if you set up the halloween-themed furniture in your room (buy them if you don't have them)
This also applies to Pixie's costume.
Chloe's witch outfit is not halloween related, but requires you to progress with her and Jessica's stories and then go check Chloe's room on a weekend.