I don't have any actual experience with this, but my guess would be Monday. I say that going on the theory that more people do a WDW trip for a week and also that they arrive/leave on a weekend. If that is true (a big IF), then I would expect more people to be doing the parks (because they just got there) on Monday but towards the end of their stay, they might decide to relax at a waterpark.
As I said - its just a theory.