Rain during the summer is very rare in the DL area. But it is hot. It's also been more humid than normal (for us) the last few weeks. Normal humidity would be under 20%, but it's been over that lately. The heat has been on for sure (a lot hotter already this summer than it was this time last summer). I'm guessing it's been low-mid 90s as a high in the last week (it's been over 100 in my area and DL is usually 10 degrees cooler). Normally, July is a bit cooler than August, with the first part of the month cooler than the latter part. August is our hottest month, July second hottest and September is actually third usually. Oth the 4th at DL I didn't even need to put my light jacket on in the evening and usually I do. On the 1st, though I did need it.
.
Expect HOT as others have said. Also, when we say that rain is very rare in the summer here - it really is. Most likely, it will not rain at all. No rain, low (compared to NY) humidiy, no thunder & lightening storms, etc. Expect a dry heat (HOT) in the day with a cooling at night. And lots and lots of 
Of course we recently tried the "go when the kids are in school" theory and went mid-week in Feb and we encountered the serious Stroller Brigade! 
If you avoid the little kid stuff until late, or during nap time, it worked out great though! That is unless you have little ones too, then I would have no idea when the best time would be, crowd wise 
