We go Thanksgiving week (Monday before - Monday after) every other year. We are going this year. It used to be (4+ years ago) that the Monday before and the Sunday after were absolutely perfect, low crowd days. As has been the general trend, the crowds have picked up (this could also be attributed to more people using fastpass now than 4 years ago when it was new, so fewer people are waiting in lines, resulting in a more crowded "feeling"). I will say our line waits have not changed dramatically (not including the new rides opening). Monday before is still good, Tuesday the crowds definitely increase, and Wednesday/Thursday/Friday are super crowded. However, I have found that there is wide fluctuation in which park people go to on which days (and I follow all the calendars, love touringplans, but on busy weeks there seems to be a lot of volatility in which park is crowded). I have also noticed that time of day can be a factor. If you want to see the MK fireworks, I agree do a dessert party, plan on staking out a spot well in advance (90 minutes probably if you want to be in the HUB), or catching it Monday night. I strongly recommend Park Hoppers to people going this week. If MK is bonkers, take the monorail to Epcot, or try one of the other parks (you can check wait times on the app, go to whichever park has the best looking wait times and make 4th FP for there on your way over. Make good use of EMH (if staying on property) and then get the heck out of dodge by 11:30am. On Thanksgiving day 2 years ago we rope-dropped MK (7am open I think), were at the gate by 6:30 - had FP for Mine Train and Splash... but we managed to do so much (Buzz, Tomorrowland Speedway, Peter Pan, Thunder Mtn, Teacups etc...) before it started getting crowded (around 10am the crowds were building). We were walking down Mainstreet toward the exit by 11:30 and the mass of people coming in was mind blowing. We went back to the hotel and swam (or went in the hot tub), and then spent the evening in Epcot (which was also crowded, but we weren't trying to do a ton of rides, just hanging out, having dinner etc...).
Friday is also bonkers. I found early mornings to be less crowded than late nights. One of the reasons is that usually MK is the only park open really late (2am with EMH sometimes). So all the night owls head there. Now, that being said, it still wasn't nearly as bad as during the day (still 30-40 minute waits for Space Mtn until about 1am, which is still too much for me). However, on a day when MK opens really early (7 or 8a) - the first 90 minutes I didn't see a single ride (except SDMT) with a legit (not posted by disney, which are often inflated first thing in the day, but an actual wait) of more than 20 minutes.
This is also a great week because the Xmas decorations are up, everything feels really festive, the resorts are beautifully decorated, and the weather is usually comfortable (it can be cold... we hit one year when it was 40 degrees!). It won't be really low crowds, but it is honestly my favorite time to go.