We have been in Dec. and Jan. several times. Honestly, we have never been able to swim on those trips. We (especially my husband) like it to be HOT outside (not just kind of warm) if we are going to swim. Yes, the pools are heated - but not enough for us to really enjoy being in the water. It doesn't really matter to us, though. There is so much other stuff to do that we probably wouldn't have time to swim anyway! However, if I had my heart set on swimming, or if I was going with somebody (like my sister) who loves to swim - I would probably go another time of year. Of course, if you are tougher than we are about being chilly, you might be just fine swimming at that time of year. I know lots of people do. We have also been in October and May and we have enjoyed the pools and the water parks on those trips.