Fairbanks has two peak seasons, and they could not be more different.
Summer (late May to mid-August) brings the midnight sun. On the solstice, Fairbanks gets nearly 22 hours of direct sunlight, and it never truly gets dark. Temperatures regularly reach 25-30°C (upper 70s to mid-80s F), sometimes higher. This is the season for Denali trips, river paddling, and the Midnight Sun Festival in late June. Mosquitoes can be brutal from mid-June through July — bring DEET or permethrin-treated clothing.
Winter (September to March) is aurora season. Fairbanks sits under the auroral oval, and the northern lights are visible on average 200+ nights per year. The best viewing is from September through March, with peak activity around the equinoxes. Temperatures routinely drop to -30°C (-22°F) and can hit -40°C or below. The World Ice Art Championships happen in late February and March.
The shoulder months of April-May and late August-September offer pleasant temperatures, fall colors (September), and fewer crowds, though some seasonal attractions may be closed.