It will be crazy-crowded. We went during the week between Christmas and New Year's and it was wall-to-wall people most of the time. It was hard to even walk around because every walkway was packed. There were CMs directing foot traffic even when there was no parade. Some lines were so long that they spilled out of the regular queue areas. Sometimes it was hard to even figure out where to get into a line because there were people standing around everywhere.
That said, it's still possible to have fun! Have a plan for Fast Passes. Get a new one exactly every two hours, or as soon as you're allowed. Prioritize your rides. You might only get on the big ones once during the trip. Figure out which ones you really want to do and plan for them. Sometimes, you just have to get into a long line and wait it out. If you have a 3-day ticket, you will get one early entry. Definitely take advantage of that. We did six Fantasyland rides one morning in the first hour, starting at 7:00 a.m. It was totally worth getting up early. The night of the 4th will be crazy with everyone staking out spaces for fireworks. If you want to see them, you'll need to get a spot early. If you're not really into watching them, stay in another area of the park and avoid crossing Main Street at all costs.
