OP, for what it's worth, if you are trying to avoid crowds (as I do), while I agree with looking at the crowd calendars to ballpark it - i.e., don't select xmas to nye week - but just go at rope drop and otherwise tour 'intelligently.'
I go annually in late August (that last week IS quiet - its much less crowded than even the week prior). I wanted to try a very low crowd time - in better weather - so I went Jan. 10-17 this year, without kids. Easy, peasy, right? Nope. Yes, the first hour or two after rope drop was very good, but you still cannot walk in a straight line without dodging people by about 11am. Not saying it was like a school vacation week, but no way did it 'feel' empty, or what I would have thought a crowd level 1 or 2 is (TP had one particular day - I think it was Wed. that week - predicted at 3 at the park I went to; the next day, in the 'how did we do' section, it said it was 'actually' a 1 (per TP). If that was a 1, I shudder to think of what a 10 is like!). During that week, as an example, I had a FP+ for the parades and Wishes at MK (I never go because I hate the jostling/waiting in position -I go lone with the kids so extra tough without 2 adults to hold the spot). At ALL 3 I had people standing close enough to feel their coughing on me, and/or squeeze in to stand in front of me, and no great spot left at that time for me to move to. And that was in the FP+ area - which was sparse compared to the rest of the area.
I guess I'm just saying that it's really just never empty at WDW. I just do the 1st 2 hours at rope drop and then anything else I get done is gravy.
I would have been annoyed if I had taken my kids out of school to experience a low crowd week, because it didn't feel that much less crowded to me - we'll stick to just doing the 1st 2 or so hours after rope drop during vacation weeks, and last week of August.
HTH!