Barcelona's Mediterranean climate makes any season visitable, but spring (April–June) and autumn (September–October) are the best windows: temperatures of 18–25°C, manageable crowds, the city in full daily rhythm. August is the worst time — peak heat and humidity, many local shops and restaurants closed for the month, and the city handed over to tourists. Winter is mild (7–14°C), often sunny, and increasingly popular with travellers wanting Gaudí without the queues.
Barcelona celebrates with intensity, and timing a visit around a festival is rewarding. The biggest is La Mercè in late September — a week-long takeover of the old town with music, fire runs (correfocs) and the towering human castles called castellers. Sant Jordi on April 23rd is Catalonia's own day of books and roses, when the Ramblas becomes one long open-air market. The Sónar festival in mid-June draws electronic-music fans from across Europe; Sant Joan on the night of June 23rd brings bonfires and fireworks to the beaches. The Grec summer festival programmes outdoor theatre, music and dance through July and August.
For traditional Catalan culture year-round, look out for castellers, correfocs and sardana dancing in the squares — these are not tourist shows but living traditions. See Catalonia Festivals for events across the wider region.