Greenland has two very distinct travel seasons, and the best time to visit depends entirely on what you want to do.
Summer (June–August)
This is peak season. The midnight sun means daylight around the clock, temperatures in the south reach 10–15°C, wildflowers bloom on the tundra, and the west coast is largely free of sea ice. Whale watching is excellent from July through September — humpbacks and minke whales are common, while narwhals and belugas can sometimes be spotted in spring. Hiking is possible almost everywhere, and boat trips to icebergs run daily from Ilulissat. Most tourist infrastructure is open and flights are most frequent. Book everything well in advance.
Winter (November–March)
Cold, dark, and logistically harder — but this is when you see the northern lights. The aurora season runs from September through April, but the darkest months give the best conditions. Ilulissat and Kangerlussuaq are popular bases for aurora hunting. Dog sledding is at its best when the sea ice is stable, usually from January through April. Temperatures in the north can drop to -30°C or lower.
Shoulder Seasons
September and October offer a middle ground: summer crowds have thinned, temperatures are still reasonable in the south, northern lights begin to appear, and the tundra turns red and gold. April and May see increasing daylight, dog sled season still running in the north, and fewer tourists than summer.
Note that the return of the sun after the polar winter is celebrated across North Greenland, typically around January 13th in Ilulissat — a quiet but genuine local event.