May and June are the sweet spot — warm but not yet too hot, with the vineyards in fresh green leaf. The first weekend of June brings the Festas de São Gonçalo, the town's biggest celebration: processions, fireworks over the river, and a full week of phallic pastry exchange.
Autumn (September–October) is the vindima (grape harvest) — a quietly festive time across the Douro and Tâmega valleys. Winters are mild but very damp and grey. Avoid August, when northern Portugal empties to the coast and many small restaurants close.