The best time to visit Arcachon is May, June, or September. The weather is warm, the water swimmable, and the town has not yet been overwhelmed by the July-August crowds that pack the beaches and make restaurants feel like waiting rooms. July and August are very busy indeed — Arcachon is one of the main summer escapes for Bordeaux, and the population of 12,000 can multiply several times over.
Outside summer, the town takes on a quieter character. The Ville d'Hiver in autumn and winter has a melancholy grandeur that suits it well — this is after all a neighbourhood built for winter visits. The oyster season runs year-round, and some argue that autumn and winter oysters are better than summer ones. Spring brings mild weather and good conditions for cycling the forest paths and sailing on the bay.