Help Finding a Story

     I once read a story I remember as being about a teenage boy dressing up as a girl as part of their Halloween costume, going to a 'feast of the Hunter's Moon'-type festival happening that day or around that time — I don't think they were testing their costume out early, but perhaps they might've been? —, met a gypsy or similar in a tent there who told them their fortune, and then got transformed into a real girl either right then or shortly thereafter before having to adjust to the change. I tried looking it up, but I couldn't find it. Does anyone happen to recall the piece I'm talking about? If so, I'd appreciate it; a link, or at least a title and author to search for, would be great.