Spring (March to May) and autumn (September to November) are the best times to visit. Temperatures are comfortable for hiking — typically 15–25°C — the wildflowers are out in spring, and the crowds at Zion are somewhat lighter than in peak summer.
Summer is hot. St. George regularly exceeds 40°C in July and August, which makes midday outdoor activity miserable. If you visit in summer, hike very early in the morning and retreat to air conditioning by noon. The heat is dry, which makes it more bearable than humid heat, but still serious.
Winter is mild and often fine for hiking — Snow Canyon and the Zion lower trails are accessible year-round. Occasional cold snaps can bring snow, but it rarely persists at the altitude of the town itself.