Cagliari and southern Sardinia have the hottest and driest climate of the island — summers are long, intensely sunny, and dry, with temperatures regularly above 35°C in July and August. This makes it ideal for beach holidays but exhausting for extended city sightseeing at midday. Spring (April–June) and early autumn (September–October) offer the best combination of good weather, manageable temperatures, and uncrowded conditions.
The Festa di Sant'Efisio on 1 May is one of the most spectacular religious processions in Italy: a statue of the plague-saint is carried by horse-drawn carriage from Cagliari to Pula (40 km south) and back over three days, accompanied by thousands of Sardinians in traditional costume from across the island. It's worth timing a visit around if you can.
Winters are mild by Italian standards — 15–18°C, rarely cold — but wet. The museums and monuments are largely uncrowded in winter and the city carries on its normal life regardless of tourism.