Last couple of times, we have been during July and August. Noticeably, the last two weeks of August/first week in September was quieter as most U.S. kids back to school. Having said that, their term dates vary greatly depending on which state they live. While some break up mid June, others break up end of May, some go back mid August while others go back beginning of September!! Asked a question about term dates over on the Universal Boards and was surprised how mixed it is over there.
Our last trip was staying on-site at the Hard Rock Hotel (Universal) last two weeks of August and it was fabulous!!! Weather was 90's (and there was a high humidity! ) But we had a fridge and used ice packs to keep the drinks cool. That is the most important aspect - plenty of drinks on you, sit in the shade and cool off in the wonderful pool - at the HRH - there is underwater music, a fun curly slide , sandy area and a wonderful beach bar to quench the thirst. More rain in July than late August we found. Also being on-site at the HRH and having FOTL (front of the line access to almost every ride) plus priority seating at all the restaurants was a huge bonus. We enjoyed the breakfast buffet every day, then casually strolled the few mins walk to Universal/IOA. Back to the pool for a while in the afternoon, then another wander late afternoon over to the parks. Every other day we went to Universal or IOA as there is so much to do and inbetween did the disney parks etc. We had a really great 2 weeks. In the evenings, you have all of citywalk, many wonderful restaurants, and automatically have priority seating - so for example - we went to the Hard Rock Cafe and there was a huge line of people waiting - we walked up to the desk, showed our room key and in less than 5 mins we were taken to a table! The kids loved this restaurant most of all. The bar staff stand on the bar singing YMCA (which is played alot on all the tv's) and make a big thing of birthdays!
This is long - can't help reliving it all, it was fab!