Sardinia has a classic Mediterranean climate: hot, dry summers and mild, wet winters. The question of when to go is mostly about how you feel about crowds.
Spring (March–May)
Many consider this the most beautiful time on the island. The winter rains leave the hills covered in wildflowers — wild orchids, cistus, asphodel — and the air smells of maquis. Temperatures are warm and pleasant (around 17°C) and you'll have most sights to yourself. The sea is still cool for swimming until May. A good choice for walking and exploring the interior.
Summer (June–August)
Peak season. Temperatures regularly reach 30°C and can hit 40°C during heatwaves. The sea is warm and the beaches are wonderful — but also crowded. August is when mainland Italians descend on the island in force, prices rise, and accommodation needs to be booked months in advance. If you visit in summer, go in June or early July rather than August. The strong summer winds (the mistral from the northwest, the sirocco from the south) make the heat bearable and the windsurfing excellent.
Autumn (September–October)
An excellent time to visit. The sea is at its warmest in early September, the crowds have thinned, and prices drop. The landscape is parched and golden. Rains begin in October but are usually sporadic at first. Late October can still be warm enough for beach days.
Winter (November–February)
Quiet and cheap. Most coastal resorts shut down entirely. Rain is frequent and the mountains can get snow. However, there's a two-week window in January — the so-called gennaietto — when the weather briefly turns warm and sunny. Cities like Cagliari and Sassari function normally year-round and have good restaurant and bar scenes. A good time for cultural travel, not for beaches.