Naantali is a summer town. The season runs from mid-June to mid-August, when Moominworld is open, the harbour fills with boats, and the old town galleries and restaurants are operating. This is when the town is at its best — and also at its busiest, particularly in July.
July 27 is Sleepyhead Day, a local tradition in which the laziest person in town is thrown into the harbour at dawn. It sounds eccentric because it is, and it draws a genuine crowd. If you are there that week, the harbour is worth visiting early in the morning.
Outside the main summer season, Naantali is quiet. Many businesses close in September and do not reopen until June. The exception is the Naantali Spa, which runs year-round. Moominworld holds a Winter Moominworld week in February, which draws families looking for a Nordic winter experience with the Moomin characters in a snowy setting.