It doesn't matter what day as long as it isn't Saturday. Sunday mornings aren't typically that bad, either. The one thing you can count on, though, is going when it is supposed to rain. Sure, the rides may go down periodically, but the time that you'll save waiting in line will more than make up for it. It is only the rare storm of the year (last year's fourth of July) that will truly affect your day. If you don't mind getting a little wet, trust me.