Spain's climate is very regional. Madrid sits on a high plateau with cold winters (2–7°C) and very hot summers (35°C+) — spring and autumn are best for visiting. Barcelona has a Mediterranean climate, mild year-round with warm summers (27–30°C).
Andalusia (Seville, Granada) is extremely hot in July–August (regularly above 40°C) — visit March–May or September–October. The Canary Islands enjoy warm, stable weather year-round and are ideal for winter sun. Northern Spain (Galicia, Basque Country) is greener and rainier.