We usually stay the longest in Sept becaue I can get a better discount (9 nights is our longest). I love Halloween at the fort, but I usually have to pay full price for a site during that time. So we only stay 2-3 nights over Halloween.
Last year, we stayed at the Fort in April... and the day we checked in was the day the last cold front came through. Talk about freezing my bejeberz off, it got into the 40's at night. By the end of the trip it was actually very nice out. It's much easier to deal with the heat cause we just use an air conditioner. I barely slept that first night.