It depends a little on what you want to see/do.
Last week in August will probably be fairly quiet in the parks, especially during the week days. Haunted Mansion will most likely be closed to have the holiday overlay installed. The park will be transitioning to HalloweenTime, but it will not be in full swing yet. Nightime entertainment, like fireworks and Fantasmic!, are usually only presented on the weekends (Fri-Sun). The parks will probably be operating on moderate-to-long hours (e.g., 9 am to 10 pm on weekdays for DL, slightly shorter hours for DCA). I would say that the weather could still be warm, but it's probably past the worse of the heat.
The week after Thanksgiving will also probably be okay in the parks, although the weekends will likely be busy. The Holiday/Christmas season will have started, which means the Christmas Parade will be running along with nightly fireworks and WoC. F! will probably only be running on weekends (based on historic operations) and the weekend (first weekend in Dec) is often Candlelight Procession which can mean high crowds in DL. Both HM and IaSW will have their holiday overlays in place. The parks will probably be operating on moderate-to-long hours (e.g., 9 am to 9 pm on weekdays for DL, slightly shorter hours for DCA). I would say that the weather can be variable, but I would plan on cool(ish) days and nights, with a chance of rain.