Winter (December–March) is what most visitors come for. Temperatures often sit between −10 and −25°C; the lakes are solidly frozen; the Northern Lights are regularly visible; and the dog sledding, snowmobiling, and ice-driving season is in full swing. Daylight is short — three to four hours in mid-winter — but the snow reflects what light there is.
Summer (June–August) is the opposite: the midnight sun never quite sets in June and July; temperatures are mild (15–20°C); and the forest, lakes, and mountains open up for hiking, fishing, and canoeing. The mosquitoes are intense around midsummer.
Avoid spring (April–May) if you want either extreme — the snow is melting and travel can be muddy.