Late June through August is the obvious season: lake ferry running, rafting trips operating, the open-air museum open daily, and the brass week filling the streets in late July. Long daylight hours — the sun barely sets in midsummer — and water warm enough to swim in.
September brings the ruska — the autumn colour that lights up the birches in front of the dark spruces — and the best month for photographing Koli. Winters are long, cold, and dark; the lake freezes solid from December to April, and locals ski and ice fish. February is the best month for the aurora here, though it appears less often than in Lapland.