The simple answer and the rule of thumb is that anytime school is in session the crowds are thinner at WDW.
Another guideline is the resort prices. Value season will be a slower time. The rooms cost less to entice you to visit at these times.
However, I am getting the impression that WDW is working on making October and November busier. The ABC Soap weekend brings in huge crowds, and the Food and Wine Festival is another draw, especially for locals who can zip in and out on a weekend day.
The Halloween party is increasing the October numbers as well.