I've been there in late July (edit: actually late June), which was pretty hot, but not ridiculous. Had one day (out of ~8) that was too hot to climb so I went on a hike and took pictures of blooming cactuses.
You can always find a shady climb any time of day.
If you haven't been there yet, you should go. It is fantastic.
EDIT: OOPS! I was there in LATE JUNE, not late July.
Might wanna check the forecast before you go and make your decision accordingly.