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If I'm not mistaken, that outfit is unlocked when you go talk to Chloe during a weekend, not sure if morning or afternoon.
You need to have progressed with both Chloe and Jessica.

Thanks, I'm glad you're enjoying it! :)

And you're not dumb at all - that's actually Chloe's special hallowe'en costume, and it's one of the "hidden" events in the game :) To trigger it you have to have done Chloe's "transformation" event (with the assassin), Jessica has to be love level 3 or higher and it has to be the weekend. Then go to Chloe's room in the morning and you'll get the special event and unlock the costume.

Hope that helps!