The peak time to visit Savonlinna is July, when the Opera Festival transforms the town and the long sub-arctic evenings stretch the day past midnight. Mid-summer days have around 19 hours of light, the lake water is finally warm enough to swim in, and almost everything in town is open. Book accommodation well in advance for the festival.
June and August are also pleasant, with cool nights and fewer crowds. Winter is a quiet, cold proposition (down to -20 C in January and February), but the frozen lakes turn into ice-skating routes and the castle is open for tours year-round. Spring (April to May) and late autumn (October to November) are the least rewarding times to visit — many seasonal businesses are closed.