I remember the times we when in September ('99 & '00), MK on Saturday was the worst!!. It was so crowded and the main reason for it being so crowded was Sat. was the only day that Spectro and the fireworks were shown. EVERYBODY was there to see that. I don't know about the other parks, but MK was crowded. Also, the locals add to the crowds on Saturday. It was such a shock to us because the rest of the week was GREAT. No crowds, no waiting it was the best and then Saturday came.
I don't know what the schedule for Spectro and Wishes is going to be when you are there, but if days for Spectro and Wishes are limited, take that into account for crowds. Crowds will be the worst on the days Spectro and Wishes are showing (this is also true for Fantasmic at MGM, if days for Fantasmic are limited). And remember I don't know about the other parks, but considering how busy MK was, I imagine the other parks were dead.
Anyone else out there know about the other parks on a Saturday??