A few years ago we arrived the Friday before T-giving and left on Tuesday. The parks were not packed, but they were busy and they got busier as we got closer to T-giving. The weekend after T-giving is quite busy, then it drops off the next week.
The good thing about going then is that most of the Christmas decorations are up - you can see the Osborne Lights, Grand Floridian gingerbread house, most of the park Christmas trees (MK's doesn't go up until after the taping of the parade the first weekend of Dec) and great atmosphere. It's much cooler than August, which is a plus. You might find it too cool to swim, if that's important to you.
If you'd like to see how Disney does Christmas, it's a pretty good time to go, but it will be a little busier at the end of the week than you may be used to.