I’m often at WDW around the long weekend holidays like Labor Day and usually do a water park somewhere in there. IMO you have to slice it pretty thin to come up with accurate/definitive statements about crowds each day, so I don’t think it really matters all that much - the weekend holiday crowd and locals will be present each day. I’d go with the flow of whatever fits in best with your overall schedule and/or weather. If anything maybe I’d pick Labor Day itself, but I wouldn’t go out of my way to reorganize my schedule around it.
My usual water park advice definitely applies - if you’re serious about a full day get there early (30-45 mins before park opening), get your spot/get organized, hit rides hard for an 1-1.5 hours, relaxing lunch, wave pool/lazy river time/sun/relax/beverages, then more rides if inclined later in the day when crowds start to drop. If you’re not the morning type you can skip the early arrival part, get there whenever, and stay until closing as those last couple of hours are usually great for ride lines as well.
Keep an eye on the posted hours, I see they currently show open until 6pm for both parks Saturday and Sunday and 5pm Monday. Highly likely those will get bumped back to longer hours closer to the date. Also in case you didn’t notice, TL has an H20 glow night event on Sunday 8pm-11pm which can keep at least some level of additional guests in the park as you get towards closing, so maybe keep that in mind in your planning as well.
Have fun!
