Avoid the water parks and any park with big thrill rides ( such a IofA and Busch) at weekends because you tend to get some locals in using their annual passes. Any major park is busy at weekends because of the number of Americans doing Fri-Mon. Though people can now arrive on any day of the week, Saturday is still the most popular arrival day and MK tends to be the first park to be visited so Sundays are busy there. Other than that, it tends to be a bit of a question of pot luck. It is not so crucial now in the days of fast passes as it used to be though.