Unless there's a huge difference in crowd levels for a particular park on a particular day, we ignore the projected crowd levels and "best days" altogether. Our touring has more to do with park schedules than crowd levels. We go where we want, when we want, with the following "rules," similar to what
@CaptainAmerica mentioned above. (A la
Pirates of the Caribbean, they're more like "guidelines" than actual rules):
- We almost always start our day with a rope drop arrival, but we avoid doing so at any park that's had a "magic morning"-type event, since the park will already be full of people when rope drop happens. (Granted, we did have to do this once, at HS, and I don't think it delayed our morning touring schedule by more than 20 minutes or so, but I'd still rather have avoided it.)
- If we have parkhoppers and there's a seasonal party at MK (Halloween, Christmas) that is going to result in an early closure every other day, we favor starting the day in MK on those days because it will be noticeably less crowded - and then we tend to avoid MK on the non-party days, when the folks with non-hopper tickets who didn't want to "waste" one on an early-closure, party day, will be scheduling their visits.
- Unless we're visiting during a high-crowd time that makes our own use of EMH a priority, we avoid starting our day at a park with morning EMH, or ending our day at a park with evening EMH.
- If we're visiting during the Food & Wine Festival at Epcot, we avoid visiting Epcot (or if we do visit Epcot, we stay out of World Showcase) on a Friday, Saturday or Sunday night, when the crowds are increased with a large number of locals who come to dine and drink. (Caveat: We have kids for whom that crowded scene doesn't offer anything. If we were visiting as a couple and were after the food and wine rather than attractions, we'd probably gravitate toward World Showcase on the weekends rather than away.)
- Since weekends are generally more crowded at WDW for every park, if we're going to have any non-park days (or waterpark days, etc.) we try to plan those for a Saturday or Sunday, so we can take advantage of the mid-week slump (lower crowds Tue-Thurs most weeks) at the parks. In fact, when planning a trip, we try to depart mid-week and come back mid-week whenever possible, since airfare is cheaper on weekdays, and this puts our preferred non-park day(s) right in the middle of the trip, when we tend to appreciate them most.