I'll tell you how...we don't notice, don't care, don't pay any attention frankly. As you said, it makes a difference to you in how the park "felt". That's perception, and everyone can perceive things differently. I'm sure there are numbers somewhere (though I know WDW doesn't publish attendance) that would support what days are busiest by strict head count, and I wouldn't doubt whatever the results are. But we pay no attention to it whatsoever, so our perception is quite different I suppose than someone who religiously looks at crowd calendars. As I said before, until a few months ago, I didn't even know such thing as a crowd calendar existed. We just went...and still do...to whatever park we wanted to on a given day. We're there to have fun and do our own thing. In all my trips to WDW, I've never seen it where I would call it a "slow" day, so I guess we just don't really pay any notice to what degree of busy it is, it's all "busy" to us. Major holidays are different...4th of July weekend, Thanksgiving, Xmas, then we have clearly noticed a difference. But the other times of year...nope, truly can't say I've looked around and said to myself "Wow, look how busy it is, it must be Monday".
Personally, and it's just my opinion, I think it's a shame that people spend all kinds of money, take time off work, do all this planning, drive/fly long distances, etc...to come down to WDW only to stay away from certain parks on certain days. To me, that really limits your trip. WDW is an extraordinarily popular tourist attraction, it's gonna be crowded. I'm paying hard earned money to go there, I'm not going to restrict myself where I go simply because of a crowd. It feels so weird for me to say that, because I truly am one who normally hates crowds. But at WDW, it's very different for me. I just don't even think about it. I couldn't even tell you how many times I've been to MK on a Monday, or HS on a Thursday, or Epcot on Sunday. Don't know. As I've said before, our "planning" for a trip is incredibly crude and simple by the standards of many here. Day of the week vs. expected crowds don't even come into the conversation. We just rotate parks daily, ensuring we hit all of them and spend more time in our favorites. We usually split it up am vs. pm, and again it's got nothing to do with day of the week and crowds. It's just for variety, and to make sure we don't do a late night in one park followed by early morning the next (kids get cranky). So yeah, it's easy for me to say it doesn't make a difference, because for us, it really doesn't.