Busiest times of the year:
-2 weeks around Christmas
-2 weeks around Easter
-Spring Break- mid February through Easter week
-mid June through mid August
Least busy times of the year:
-September, October, November (except Thanksgiving week) and the first two weeks of December
-January 2 through mid February
Some say there are no "slow" times at Disney anymore- there are always some people around. That "only ones in the park" feel is pretty much a thing of the past, but there are times that are MUCH less crowded than the peak times.