I've never been to Kings Island on a Saturday in the fall (In the last 15 years, anyway). I'm guessing Sundays are the preferred day during the fall, but are Saturdays jammed with guests? Next Saturday (9/21) will be the first publicly open day in two weeks (I know they'll be open Fri. night), so I'm guessing there may be a backlog of guests.

Anyone have any insight on anticipated crowd size for 9/21?

I believe the first weekend of Haunt is generally "less" crowded than the rest. However, I'd still expect the park to be very crowded next Saturday. It'll likely just get worse and worse each Saturday though.

Just remember that saturday 9/21 the park opens to the public at 7pm, due to a private function during the day. So it will be interesting to see what the crowd size will be during the night.