Kiruna has two distinct visitor seasons, and your choice entirely depends on what you want to see.
Winter (November–March) is peak Aurora Borealis season. The polar night — roughly 28 days of continuous darkness around the winter solstice in late December — creates ideal conditions for Northern Lights viewing. Temperatures routinely fall to -20°C and lower, but the cold is dry and manageable with proper clothing. This is also when dog sledding, snowmobile safaris, ice fishing, and the Icehotel in Jukkasjärvi are fully operational. Book accommodation well in advance for December and January; the Icehotel in particular sells out months ahead.
Summer (late May–mid-July) brings the midnight sun: continuous daylight for about two months. Temperatures are mild (10–20°C), the landscape turns vivid green, and hiking, fishing, and whitewater activities come into their own. Midsummer (late June) is a major celebration across Sweden, and Kiruna observes it with particular vigour given the symbolic importance of sunlight this far north.
Spring and autumn are transitional and offer fewer draws. April and May see the snow melt and trails become muddy; September and October bring the first dark nights and early aurora chances, but full snow infrastructure isn't yet in place.
The annual Kiruna Snow Festival in late January or early February features ice sculpture competitions, reindeer racing, and events across the city — a lively counterpoint to the deep cold.