Portugal has a mild climate, and most of the country is worth visiting year-round — but the timing makes a real difference in cost, crowds, and what's possible.
Spring (March–May) is the best all-round season. Temperatures are comfortable, wildflowers carpet the Alentejo and Minho, and tourist numbers are manageable. May in particular is hard to beat: warm enough for the coast, not yet overrun with summer visitors.
Summer (June–August) is peak season. The Algarve and Lisbon's waterfront get seriously crowded in July and August, prices rise sharply, and the interior can be ferociously hot (40°C or more in the Alentejo). That said, the long days, the beach culture, and the festas santos populares in June — especially Lisbon's Festa de Santo António on June 12–13 and Porto's São João on June 23–24 — make summer hard to avoid if you want to see Portugal at its most alive.
Autumn (September–October) is another excellent window. The coast is still warm, the Douro Valley is in harvest, prices drop, and crowds thin after mid-September. The water stays swimmable on the Algarve into October.
Winter (November–February) is the off-season except for the Algarve, where mild days still bring visitors. Lisbon and Porto have their wet spells but plenty of dry, sunny days. Madeira and the Azores are accessible year-round, though winter brings more rain (and impressive surf at Nazaré). Budget travelers will find the best deals October through March.