October through April is the visiting season. October and November are ideal: warm but not dangerous, the crowds from spring haven't arrived, and the light is extraordinary. March and April bring wildflowers after a wet winter — in exceptional years (roughly once a decade) the valley floor turns yellow with desert gold. February and March are the busiest months.
Summer is serious. From June through September, daytime temperatures regularly exceed 45°C and have hit over 50°C. The park doesn't close, but people die here every summer — usually from heat stroke after underestimating the walk back to their car, or from breakdowns on remote roads. If you go in summer, stay in your car with the AC running, and confine outdoor activities to before 8am and after sunset.
December and January can be cold at night — below freezing at higher elevations — and the high mountain roads may have snow. The valley floor stays mild, but pack layers.