Dinan is good year-round, but summer — especially July and August — brings the most tourists and the warmest weather. The Fête des Remparts takes over the town on the third weekend of July in even-numbered years (2026, 2028, etc.): it's a large medieval festival with jousting, period costumes, markets, and street performers. If you want to see it, book accommodation well in advance. If you want to avoid the crowds, avoid it.
May and June offer mild temperatures, less crowding, and the market gardens of Brittany at their best. Autumn brings good light for photography and near-empty streets. Winter is quiet — some restaurants close — but the stone town looks handsome in low light and you can have it largely to yourself.