Barcelona has an excellent Mediterranean climate. Summers (June–August) are warm (27–30°C) and sunny with sea breezes keeping it bearable. The beach season runs May–October. Winters (December–February) are mild (10–14°C) with occasional rain — ideal for sightseeing without crowds or heat.
Spring (April–May) and autumn (September–October) are perfect: warm enough for beach visits, cool enough for walking, and with manageable tourist crowds. July–August is peak season with maximum prices and crowds at major sights. Easter week brings large processions. September's La Mercè festival (city's main festival) is a highlight if you're visiting then.