Spring (April–May) and autumn (September–October) are the best times: mild temperatures, manageable crowds, and the full range of sights and restaurants accessible. April brings the Vinitaly wine fair, which fills the city; book accommodation early if visiting then.
Summer (June–August) is high season and hot. The opera festival at the Arena runs late June to late August and is the main reason many people visit; performances are spectacular but tickets are expensive and the city is at its most crowded.
Winter is cold but the centre is lively, with a good Christmas market on Piazza Bra. The Bacanal del Gnoco carnival (the Friday before Shrove Tuesday) is one of Italy's oldest carnivals and worth knowing about.