Midsummer (June to August) is Mariehamn's peak season, when the Swedish and Finnish ferry passengers are most numerous and the cycling and outdoor activities are at their best. The days are extremely long — Midsommar in late June involves essentially no darkness — and the town fills for the Åland Game Fair in summer. The Midsommar celebrations themselves are a genuine event worth timing a visit around.
May and September are pleasant shoulder months with fewer visitors, manageable prices, and good cycling weather. Winter is quiet and cold; the ferries continue to run but many tourist facilities close. The northern lights are occasionally visible in the darker winter months, though Mariehamn's latitude (60°N) puts it at the southern edge of regular aurora country.