Good advice from Mark and Noseoil.
The bottom of the Grand Canyon is Sonoran Desert zone, the rims are alpine. In June, July and August the radiated heat from the desert pavement in the Sonora Desert can result in air temperatures that reach (or exceed) 140 degrees F. In addition, you won't see any saguaro blossoms during the summer - fruit, maybe.
In general, the best time of year for saguaro blossoms is March or early April. However, the blooming time in a specific area will be determined by that area's macroclimate.
Born there, been there, done that.