Bolzano sits in a deep valley that traps heat in summer, making July and August genuinely hot — temperatures regularly exceed 30°C in the city, which can be oppressive. The saving grace is the cable cars: you can be at 2,000m within half an hour. Spring (April to June) and autumn (September to October) are the best times for the city itself: mild temperatures, good light, and fewer tourists.
December brings the famous Christmas market on Piazza Walther, one of the most atmospheric in the Alps. The market runs from late November to the Epiphany; the square fills with wooden stalls, the smell of mulled wine and roasting chestnuts, and the Duomo is lit impressively at night. The ski resorts of the Dolomites are within easy reach in winter; the city itself sees little snow.