Hanko has a short, fierce summer and a long quiet winter. Late June through August is the obvious window — long bright evenings, swimmable Baltic, the villas open for rent, the restaurants firing on full cylinder. Hanko Regatta in early July is the social event of the year and books out the town months ahead.
Late May and early September are the connoisseur's choices: the light is still long, the migration brings birders to Uddskatan, and the spa-town atmosphere persists without the queue. Winter (November through March) shuts most of the visitor infrastructure, but the empty wooden villas in snow are a quietly spectacular sight if you have winter clothes and your own transport.