I have to cast my vote for early December. We've been going the first or second week in Dec. (whatever week is not Pop Warner week) for the past 4 years now? This past December was the first time at The Fort and we LOVED IT! The campground is amazing around Christmas.
We generally like anytime from late Sept - April excluding Thanksgiving, Christmas, New Years, President's Day, and Spring Break. The weather is pretty good and the crowds are low.