Summer (mid-June to mid-September) is the obvious time to visit if Glacier National Park is the goal. The Going-to-the-Sun Road is usually fully open by early July, and this is when the Flathead Valley is at its greenest and most active. Kalispell itself is pleasant in summer — warm days, cool nights, and long evening light. Expect crowds at Glacier; arrive early or stay late in the day.
Late spring (May to early June) and early autumn (mid-September to October) offer the best combination of weather and manageable crowds. Wildflowers peak in June, and fall brings larch color to the mountains. Winters are cold and snowy. The town functions year-round, but Glacier access is severely limited from October to May.