This was a kids’ picture book from the late 70s or early 80s (probably 4/5th grade reading level, 32ish page length, not just a simple picture book but not chapter or young adult) level. The premise is that two friends (I think they were two girls, might have been just one) show up at a house for a Halloween party, They think it’s the right house, but upon getting inside, are met with another girl who I *think* was wearing some kind of a spider costume – she might even have been named “spider”. They proceed to go to the basement (which I don’t remember being a scary development). At some point they swap costumes (possibly against the will of the girl who showed up to the house).  The girl who came to the house doesn’t want to be there, and eventually finds a way to escape/leave. Important to underscore that even though it sounds a bit this way,the tone of the book isn’t scary or sinister – more just a vibe of “strange” and “oddball”. A fun little Halloween story that I vaguely remember and am desperate for help locating!