As someone who loves to stay up late and sleep in later...zzzzz.... sorry, fell asleep there for a moment. Anyway, I've found that going during the off season means you can pretty much go to any park on any day and not worry horribly about the crowds. Personally, I like that because I can stay at Pleasure Island until the wee hours, sleep in and not wake up with that awful "shoot, I should have been at the park hours ago to avoid the crowds!" feeling.
I generally go from the Saturday after Thanksgiving until the following Saturday. The first day or two, Saturday and Sunday, are sometimes crowded, but the rest of the week, things really seem to level off. I've walked on to many of the most popular attractions, no matter which day I go.
I will say that even in the off season, you want to hit the water parks during the week, rather than on the weekend. Blizzard Beach, my favorite, fills up fast! But on, say, a Wednesday, it gets crowded, but not overly so.