Don't know about the busiest days, but I suspect Friday - Sunday. We just got back from a "water parks only" trip and went to BB on Monday and TL on Tuesday. Both days were manageable, and I think early in the week is probably the best since people coming in on weekends head straight for the theme parks and save the water parks for later in the week. Also, I highly recommend being at the water parks about 30 minutes before stated opening. This strategy worked great at both parks. We were among the first in and were on all the slides and attractions we just had to be on before the lines started. When the lines built up, we either decided if we really wanted to wait or we headed for the wave pools and lazy rivers. (We left both days around 1:30. This was just in time because storms came up both days right around 2 - 3.)
Julie
