Zion is on every list of America's busiest national parks, and the main canyon now sees five million visitors a year. Spring and autumn are the windows. April to early May gives green cottonwoods, modest water levels in the Narrows, and bearable temperatures; mid-September to early November brings cooler hiking weather and golden autumn colour along the river. Summer is hot — 38°C and higher in July and August — and crowded; winter is quiet but the high trails are icy, and the Narrows are often closed.
Plan around the lottery for Angels Landing and around the snowmelt for the Narrows. The shuttle starts running in late February or early March and continues into late November. Avoid late July through mid-September if you can — the flash-flood risk in the slot canyons rises sharply.