Note that I didn't say
SLOW season, I said
slowest.
At least one widely-used crowd calendar says that it is, especially during peak hurricane season in September (which, granted, does not usually mean significantly cooler temps yet.) Thanksgiving week, of course, is a season unto itself, and some people think that Jersey Week is as well. Parts of January and February are slow also, but that period is more broken up by two long weekends, and there are also the Australian Summer holidays in January and the UK/Ireland half-term holidays in February.
http://allears.net/walt-disney-world/wdw-planning/historic-attendance-levels/ (they base this on a simplified avg. of the TouringPlans crowd calendars)
This year, of course, the planned opening of GE will increase crowds at the Studios.