Definitely BEFORE President's Week. We went the 9-16.....we left the Thursday before the actual President's Day. The crowds really started to pick up our last day or 2. Also, the rates will go into Peak instead of Value once you get to President's Week and I don't think they go back down for a while after that.
We loved going down in February. Crowds were very, very manageable. The weather is a little unpredictable. But just make sure to bring a variety of clothes. One BIG plus is that Feb is the driest month of the year. We had one day where it rained a lot, but it was just one day as opposed to DAILY rain showers that happen other times of year. Usually we started out the day w/ sweatshirts w/ t-shirts underneath. By mid to late morning we were very comfortable w/o the sweatshirt. We wore a lot of pants, but there were plenty of people wearing shorts. If you don't absolutely HAVE to go to the waterparks or beach, then you will be very happy going this time of year. It did manage to warm up to the low 80's our last day so we did get to use the pool then.