I don't subscribe to any Touring plan, and all of the info I have gotten has either come from these boards or from googling on my own, so I suppose that I am safe sharing what I have found (apparently certain Touring sites will burn down your kids tree houses (with your kids inside) if you even think of sharing some of their privilledged info

).
But what I have discovered (and I really don't know from which sites, I have seen so many) is that the WORST days to visit each park are:
Magic Kingdom - Sunday, Monday and Saturday
EPCOT - Wednesday and Friday
MGM - Tuesday, Wednesday and Saturday
Animal Kingdom - Monday, Tuesday and Friday.
Now, as to how accurate this info is, I'm not sure. We will be going to WDW for the first time in many years in Dec (2007). So take this for what it's worth, but I am planning my trip around these days.
By the way, if you hear a loud scream, it might be me getting flamed for unknowingly revealing secret information.