The summer — June, July, August — is the obvious time to visit. The harbour comes alive, the cycling routes are at their best, and the long Danish evenings stretch on past ten o'clock. The major events of the calendar are the Sønderborg Tilting Festival in early July (the Ringridning, a horseback skill contest unique to the region) and the open-air concerts around the castle through July and August.
The shoulder seasons of May and September are cooler but still good for walking and cycling. Winter is dark, often grey, and most of the harbour businesses close — visit then only for the castle museum and the battlefield, both of which stay open year-round.