Viana do Castelo's best month is August, when the Festas de Nossa Senhora d'Agonia — Our Lady of Agony — transforms the city for three days. Processions fill the streets, women wear the full Minho folk costume with layers of gold filigree, and fireworks go up over the river. Book accommodation months ahead for the festival weekend.
The rest of July and August are warm and relatively dry; June and September are the most comfortable months for visiting without crowds. The northwest coast is wetter than the rest of Portugal — rain is possible any time from October to April, and the wind off the Atlantic can be sharp. Winter is quiet but not unpleasant; the cathedral and basilica are accessible year-round, and the restaurants are welcoming when the weather turns.