Alta has two distinct seasons that attract very different visitors.
Winter (November–March) is the time for Northern Lights, dog sledding, snowmobile trips, and the ice hotel at Sorrisniva. The aurora is visible on clear nights from late September, but the darkest skies — and therefore the best viewing — are in December and January, when the sun doesn't rise at all for several weeks (polar night). The downside is temperatures that regularly drop to -20°C or colder, and wind that makes it feel worse.
Summer (May–August) brings the midnight sun — continuous daylight from late May through late July — and the Alta River's salmon season. Birch forests turn vivid green, the plateau is hikeable, and the rock art trail at the Alta Museum is at its most accessible. The salmon fishing season peaks in June and July. Summer crowds are lighter than you might expect; Alta is genuinely remote.
Spring and autumn are transitional and less visited, though the Northern Lights return from late August and September can be beautiful.