Your travel dates pretty much depend on what kind of sport you want to do. Ski (and snowshoe and Nordic) season generally starts around Thanksgiving and lasts until April (though it’s not uncommon for lifts to continue running well into June, depending on the amount of snowfall), with an average of 300 inches of snow a year.
The high-country-hiking window is short (July to early October), but storms can blow in anytime, especially up around the 12,000-foot passes. Same goes for mountain biking—the valley dries up in late May. The earlier part of the month is considered “mud season” by the locals. The snow is melted, the hills are muddy, and the spring flowers haven’t bloomed yet.
Independence Pass, on Highway 82 at 12,095 feet, usually opens around Memorial Day (it’s closed due to avalanche activity in winter) and signals the true beginning of road-biking season. Summers are dry and pleasant, with highs hitting 80; afternoon thundershowers can look nasty, but they’re usually quick. If you want fall color, aim for mid- to late September.
Name a category and Aspen has a top-notch gathering for it—Aspen Summer Words, a literary festival (June); the Aspen Music Festival and School (June through July); an enormous food and wine festival (June); a five-week dance series starting in July; outdoor concerts over both the Memorial Day and Labor Day weekends with Jazz Aspen Snowmass; and, of course, a bunch of ski and snowboard competitions throughout the winter.