The two main seasons are the polar night winter (late November to mid-January, when the sun does not rise) and the midnight sun summer (late May to late July, when it does not set). Both are reasons to come; the in-between months are quieter and cheaper.
December to March is peak winter — snow, husky rides, Northern Lights, skiing. Christmas and February school holidays are the busiest weeks. Temperatures regularly drop below -20 °C and can hit -30 °C. The aurora is most often seen from September to early April; clearer skies in February and March make those months the best of the season.
Late June to July is the midnight sun, with the sun above the horizon 24 hours a day for several weeks. Hiking, fishing, and berry picking peak in July. Mosquitoes are at their worst in the same window — bring repellent.
Autumn (mid-September to mid-October) is the ruska, when the birch and dwarf shrubs turn red, orange, and gold across the fells. Many hikers consider this the most beautiful time to walk in Lapland.
April to mid-May and November are the shoulders — the snow is melting or not quite arrived, many activities have stopped, and the village is sleepy. Avoid these months unless you specifically want quiet.