Jeez Moishe I've boiled there in late May. The weather is still changeable. In the Totonaka park in 1987, I recorded more than a dozen days in a row in mid June that reached 100 in the shade. July is even hotter, and around the end of July and the first of August the temperature goes down a little but steam-bath humidity rolls in. That time of year, shady parking is what I crave most.
Anyway, the sooner the better. You aren't going to find it cool there. The water temps might be a little on the brisk side but the sun will be bright and warm.