My husband is a teacher, so we went back and forth between spring break or summer for our first trip this past year. We decided on summer, but went in mid-June; June 17-24 (after all of the grad nights were over). We LOVED it. The parks were open early/late, all of the entertainment was going on daily (parades, fireworks, etc.), and the weather was amazing. Chilly/cool in the early mornings/evenings, but really nice during the day. I think the week or two after we left, the temp jumped to triple digits. So not sure if we got lucky with the cooler weather. I know some people talk about "june gloom" so maybe that was part of it too.
ETA: This ignores the impact SWG will have next summer!