Viterbo sits at 300–400 metres above sea level on a volcanic plateau, which makes it noticeably cooler and less humid than Rome in summer — a genuine relief. Spring (April–June) and autumn (September–October) are the best times to visit: mild temperatures, clear light on the stone, and the surrounding countryside at its most beautiful.
July and August are warm but manageable; the city is far less crowded than Rome or the Tuscan towns at the same latitude, and the nearby lakes provide relief from the heat. Winter is cold and quiet; the medieval streets can be atmospheric in low season but some restaurants and attractions reduce their hours.
Macchina di Santa Rosa (3 September): the most important event in Viterbo's calendar and one of the great popular spectacles of central Italy. Each year a massive illuminated tower (the macchina), now standing over 30 metres tall, is carried through the streets on the shoulders of about 100 men (facchini) in a procession celebrating the city's patron saint. The event takes place at night, with the tower lit by thousands of candles and electric lights. It is genuinely spectacular and worth planning a trip around; book accommodation weeks in advance.
Easter and Holy Week draw significant numbers of pilgrims to the cathedral and the Papal Palace. The Feast of Corpus Christi (June) sees elaborate floral carpets laid through the streets of the old town.