Spring (March to May) and autumn (September to October) are the preferred seasons. Temperatures are comfortable for hiking, the light is exceptional, and flash flood risk is lower than in summer — though it is never zero. The desert blooms in early spring, and the canyon walls change colour dramatically through the day.
Summer is hot and dangerous. Afternoon thunderstorms are frequent from July through September, and flash floods in the narrows can be fatal — the canyon offers no escape once you're inside. The BLM rates daily flood risk and closes the canyon to hikers when the risk is high. Check the forecast obsessively before entering, and trust the closure.
Winter is cold, with snow possible and standing water in the canyon that may freeze. Some hikers prefer this season for solitude; you'll likely have the canyon to yourself. Wetsuits or dry suits are useful if you plan to wade through standing pools. Permits are still required.
Permits sell out months in advance for spring and autumn weekends. If you're flexible on dates, weekdays in shoulder season give the best combination of good weather and availability.