As you probably know, we have quite heavy snowpack this year in the Sierras.
This means that the mosquitos won't thin out until late August. I think that any start date in July will mean heavy mosquito load. If you care about bugs, then the later you can go the better. July will have heavy bugs. September will be bug free, but few flowers. Early August will still be buggy, late August might or might not be bug free. If you don't care about bugs, then any July date will have great flowers.