Kemijärvi has two distinct seasons worth coming for. Summer (June to August) brings the midnight sun — from late May to mid-July the sun doesn't set at all. The lake is warm enough for swimming, the hiking is excellent, and the forests are thick with blueberries and cloudberries. Mosquitoes are fierce in July but manageable with repellent.
Winter (December to March) is the other main season: snow from November, a frozen lake by December, and the best chance of seeing the Northern Lights, which are visible on clear nights from September through March. Temperatures drop to -20°C and below, so dress in proper layers. The town goes quiet and beautiful in a stripped-back way.
September brings spectacular autumn foliage — the Lapland ruska — when the birch trees turn gold and the fell vegetation goes crimson. It's a brief but extraordinary display and one of the best times to hike in the national park.