Anchorage has two seasons that matter for visitors: summer and winter, with brief transitions either side.
Summer runs from late May through early September. June and July are the best months — daylight is functionally endless (the sun barely dips below the horizon in late June), temperatures are mild (mid-60s to low-70s F), and the wildlife is out. August brings the salmon runs and the first hints of autumn colour in the mountains. September is increasingly cold and many tourist services start winding down. Mosquitoes are a real problem June through July, particularly outside the city.
Winter runs October through April. December and January are the darkest months — five hours of daylight at the solstice. Temperatures average 15–25°F, occasionally dropping to -20°F in cold snaps. The compensation is a real chance of seeing the Northern Lights on clear nights (better away from city light pollution), plus skiing, dog sledding, and the Iditarod ceremonial start in early March. The Fur Rendezvous festival livens up February.
Spring and autumn are brief. "Breakup" — the muddy thaw — lasts a few weeks in April and May; September brings the first frosts and stunning yellow and red mountain colour for a couple of weeks.