Vigo has the typical Atlantic Galician climate: mild year-round but wet, with the rain arriving in earnest from October through March. The city is jokingly called "the city of rain" by Spaniards — average annual precipitation is around 1,500 mm, much of it falling as persistent drizzle. This is the flip side of the green countryside and the fresh seafood.
June through September is the best time to visit for beach weather and the Islas Cíes ferries, though July and August bring Spanish domestic tourists in large numbers. May and September are the sweet spot — reliably dry, warm enough for the beach, and quieter. The local festival of San Xoán (Midsummer) in late June is one of the livelier events in the Galician calendar, with bonfires on the beach.
Winter is genuinely mild — rarely below 5°C — and the seafood season continues year-round. The city functions perfectly well in winter, with good value accommodation and locals getting on with their lives rather than performing for tourists.