Spring (March through May) is the best time for Sonora. Wildflowers cover the foothills, the Tuolumne River runs fast for rafting, and daytime temperatures are comfortable in the high sixties and seventies. The hills are green before the summer heat burns them off.
Summer gets genuinely hot — daytime temperatures often reach the mid-90s Fahrenheit (35°C) in July and August. The Gold Rush towns and museums are at their most crowded on summer weekends, with families on school-holiday road trips. If you visit in summer, start early in the morning and retreat indoors during the afternoon.
Autumn brings cooler temperatures, the hills turn gold (which feels appropriate), and the tourist crowds thin considerably. October and November are underrated months. Winter is mild at this elevation — snow is possible but doesn't usually stick in town — and the historic sites are quiet and atmospheric.