December to April is the ski season, with reliable snow at this altitude. February and March generally have the best combination of snow, daylight, and slightly milder temperatures; January is colder, darker, and quieter. The ice music festival adds a draw in late January or February.
June to September is hiking season on the Hardangervidda. July and August have the most reliable weather and the longest days — light enough to walk almost around the clock at midsummer — but also the most mosquitoes. September brings autumn colours and quieter trails, with frosts at night.
May and October are the shoulder seasons: the snow is melting (or starting to fall again), some lifts and huts are closed, and many businesses take a break. Pleasant for quiet visits but limited for activities.