Yellowstone in September

Last year we left Indiana the Friday before Labor Day. We stopped in Wall SD and then Cody for a few days. We arrived in Yellowstone the Wednesday after Labor Day. We stayed four nights in the park and did have reservations at Canyon campground. I know our campground was full. Not sure about the others.
This was our first trip there so I don't know how crowded it is during the summer months. While we were there it didn't seem to bad. We only got stuck in traffic once and that was due to buffalo in the road. The only place we couldn't find parking was the trailhead for Mount Washburn. Everywhere we went it easy to park and the trails weren't crowded. It was an awesome trip.