Bilbao has a famously wet, Atlantic climate — the locals call their city "Botxo" (the hole) for the bowl of hills that traps clouds and rain. The city averages 130 rain days a year, spread fairly evenly through all seasons. That said, summer (June to September) is the driest and warmest period, with average highs around 24°C and occasional warm spells. Spring and autumn are mild and often pleasant despite intermittent rain.
The Semana Grande (Aste Nagusia in Basque) in mid-August is the city's main festival week — nine days of concerts, fireworks, street performances, and bullfights that draw enormous crowds and fill every hotel. Book accommodation months in advance if you plan to visit then. For calmer sightseeing, May–June or September–October are better choices. Winters are grey but mild (rarely below 5°C) and the city quiets down for proper exploration without crowds.