Summer crowds die down during the last week or so of August as kids get ready to return to school, then the crowds evaporate for the whole month of September. WDW is a ghost town in September.
Crowds are reasonably thin in October, though a little heavier than September. October weather is much better than September, though; it's a little cooler but still hot enough for swimming, and the humidity drops, and the rains die down a lot.
The first two weeks of December are usually very empty, but crowds have been getting a little heavier as word has spread over the internet. Crowds during December build slowly until Christmas week, when they explode into the heaviest week of the year.
But one or two days after New Years, the crowds evaporate again, and WDW is deserted once more the rest of January and part of February.