Spring (April–May) and autumn (September–October) are the best times to visit. The weather is warm without being brutal, and Catania is less crowded than in July and August. The city is also a better base for Etna in these months — summer heat makes the upper slopes uncomfortable and afternoon cloud frequently obscures the summit.
July and August are hot — often over 35°C — and humid. The city empties of locals who leave for the coast or the mountains, while it fills with tourists. If you visit in summer, start early and slow down at midday.
The feast of Sant'Agata (3–5 February) is one of the most intense religious festivals in Sicily: a candlelit procession through the city that runs through the night, drawing hundreds of thousands of participants. It is genuinely extraordinary if you're in the city for it.
Winter is mild by northern European standards but can be wet. Etna is snowcapped from November onward and skiing is possible on the northern slopes.