Spring and autumn are Zion's best seasons, with comfortable hiking temperatures and softer light on the canyon walls. April, May, September, and October are especially good, though spring runoff can close The Narrows when the Virgin River is too high.
Summer is busy and brutally hot in exposed areas, but it has long days and reliable access to most trails. Winter is quieter and can be magical after snow, though ice affects shaded trails and services are reduced.