We always go in October. It's likely to be hot and humid, so you'll probably be okay with shorts and t-shirts. I usually pack shorts, one or two pairs of capris, and wear a pair of long pants on the plane. Bring a light layer (jacket, light sweatshirt) to wear in the air conditioned buildings, especially in the evening. The contrast between hot/humid outside, and heavy a/c inside, can be a little bit of a shock! I've shivered in restaurants before.
But I'd recommend that you check the weather forecast right before you go, just in case. I went to WDW in October 2005, and the second day we were there, Hurricane Wilma went through. We weren't impacted too much (parks closed for 1/2-day, had to stay in the hotel until the storm blew through), but the weather cooled off significantly after the storm went through. It was even too cool later in the week to sit by the pool comfortably. Was glad I had some capris and a sweatshirt.
So you just never know - just check the forecast as you're packing, and you'll be able to tell what to do.