Saint-Malo is heavily seasonal. July and August are crowded, expensive, and require advance booking for accommodation. The ferry arrivals from Britain and Ireland mean the crowds are particularly noticeable. That said, the weather is at its best, the boat trips to the islands are running, and the beaches are at their most lively.
June and September are significantly better: the weather is nearly as good, the crowds are lighter, and prices drop noticeably. Spring (April–May) is pleasant for walking the ramparts, though the water is still too cold for swimming and some boat trips haven't started. Winter is quiet — the Intra-Muros feels genuinely like a working town rather than a tourist set. The New Year celebrations and the Route du Rhum sailing race (every four years in November) bring brief bursts of visitors.