I have been doing a lot of research for my upcoming trip (September) and it really seems like the day of the week that's best for each particular park varies regularly. You have to take into consideration the crowd, whether or not there are EMH that day, opening and closing times, etc.
Suggested resources -
The Unofficial Guide (buy it or check it out from the Library)
Tourguidemike (subscription service)
Allearsnet (free)
Hope you find what you're looking for!