Aspen: Strategies: When to Go: Summer
Summer
If you absolutely must avoid the snow, wait until July 4 to visit Aspen. June can be, and often is, warm and beautiful, but a sunny morning can quickly turn into an afternoon spring blizzard that has locals slogging through snowdrifts in their sandals. By July, mild temperatures usually prevail, rarely rising above the low 80s during the day or dropping below the mid 40s at night.
Typically, the Aspen cultural season kicks off with the Food & Wine Magazine Classic in mid June and ends, along with the Aspen Music Festival, sometime during the third week of August. Come in September, and you’ll find crowds at an ebb and weather that may be the best of the entire year. To catch the changing of the leaves, you should visit between the end of September and the middle of October, although a surprise freeze or snowfall can move prime viewing up a week or two.
— Diane Tegmeyer 05/16/2007