As daybreaker points out, there is no particular season which is completely free of crowds, because even in the middle of February the parks are packed for the half-marathon. Check the calendar at http://www.undercovertourist.com/planning/when-to-visit.html for the specific days you have in mind. For reasons that aren’t clear to me, Disney’s official site doesn’t list scheduled closures far in advance, so consult http://www.wdwinfo.com/wdwinfo/rehab.htm to see if any of your personal E-tickets are going to be closed.
The absolutely smallest crowds are immediately preceding and following a hurricane, but that won’t help long-term planning.
The best time, generally, is when kids are still in school. This won’t guarantee a less crowded park, but it helps. This means late Jan/early Feb, as long as you don’t go around President’s Day, late April/early May, late Sept through early November (though this is also Epcot Wine and Food festival time leading to larger crowds in EPCOT), and early December.
Early December is my favorite, because the parks are all decorated for Christmas, the weather is cooler, there are fewer rides down for maintenance (which usually happens in January-March), and it is usually value season with regards to hotel rates. And the crowds are usually pretty light, because parents are less likely to pull their kids out of school with one to two weeks left before finals.
The one thing to watch with early December is when the Pop Warner Football / Cheerleading championships are. I didn’t find that it added to the crowds any, but some people find the large groups of pre-teenagers annoying. I still feel like this isn’t enough to knock early December from my top spot, as the other times of the year have their own pitfalls as well (Jan/Feb = maintenance time for lots of rides, May = hot, Sept/Oct = Food & Wine, which would be fun, but is an extra cost and crowded).
You can also check out http://www.touringplans.com, which is a subscription service with TONS of great info like crowd level calendars (they employ a full time statistician), and touring plans to hit as many rides as possible with as little a wait as possible.
We went to Disney this March (beginning of the month) and it was almost empty (5 minute lines for almost every attraction). This was before the school break.
I followed this link that gave me the dates to avoid (see the site for reasons):
Credit:stackoverflow.com‘
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