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While the room is decorated with all the Hallowe'en stuff, visit the Manor Dungeon in the EVENING (must own the Manor, and have bought the Dungeon upgrade)

With the decorations up, you then have to visit the manor's dungeon.

Provided you repaired it, of course.

So after decorating your room with the Pumpkin, Coffin and Halloween picture, wait until the evening and then head down into the dungeon (assuming you've unlocked it). The event with Jess and Gwen will play and you can choose Jess or Gwen. Play the event again the next night and choose the other girl.