First of all, smart not going in July. I live in Orlando and I pretty much steer clear of the theme parks during that month.
August is smarter, but still extremely crowded. If you can, go in September (best month weather-wise and crowd-wise), but if you can't, try and get in the last couple of weeks of August.
For your date, you'll still have to get in the parks when they open to ride a majority of the rides, and expect some long lines, but most of the lines die down when the sun goes down anyway - even in the summer. There isn't a 'super cool new ride' (except for Soarin', if you count that new), so there won't be one that's like 6 hours or something crazy like that (Soarin' does get long, though!).
The weather... well, I used to walk home from school in the August sun, so my opinion may be biased, haha. Sometimes you'll get a breeze, but bring a fan and sunscreen if you can. Hot and humid, usually. It's good for the waterparks, but not really for the lines. Luckily, most of the lines are in the air-conditioning, so that helps!
x
Source(s):
Live in Orlando, been to WDW millions of times in all months. :)