The Basque coast is wet. San Sebastián gets around 1,500mm of rain a year, falling in fine Atlantic drizzle (the Basques call it sirimiri) for much of the year. Even in summer it can rain for days at a stretch. The best weather is generally May–June and September–October, when the temperature is pleasant, the rain less persistent, and the crowds manageable.
July and August are peak summer: the beaches are packed with Spanish and French tourists, accommodation is expensive and often booked out, and the city is lively but exhausting. September is the best month: the San Sebastián International Film Festival runs for ten days mid-month, turning the city into a red carpet event with screenings, parties, and the surreal spectacle of film stars wandering the pintxo bars; the weather is usually still warm enough for the beach; and the crowds begin to thin.
The Semana Grande (Great Week) festival in mid-August brings concerts, street parties, and fireworks over the bay — spectacular but very busy. Tamborrada, on 20 January, is the city's most distinctive festival: at midnight, thousands of participants in military and cook costumes march through the streets playing drums all night. It's extraordinary and uniquely San Sebastián.