Keystone has two clear seasons separated by a muddy shoulder in late spring and a brief lull in autumn.
Winter (November–April): The ski season typically opens in mid-November — often the first major Colorado resort to open thanks to aggressive snowmaking — and runs to mid-April. The resort sits at 9,280 feet at base level, which generally means cold, reliable snow. January and February are the best months for powder: snowfall is highest and temperatures keep conditions fresh. March brings longer days and corn-snow conditions on sunny faces — pleasant skiing if not always powder. The Christmas–New Year period is the most crowded and expensive time to visit; book well ahead.
Keystone is unique among Colorado resorts for its night skiing operation. Nearly 1,700 acres are lit until 8 pm, which makes it a realistic option for Denver residents who come up after work. Midweek evenings are dramatically uncrowded.
Summer (June–September): July and August are the peak summer months. Afternoon thunderstorms are common — typical of the Colorado high country — but mornings are usually clear. Temperatures at the base are in the 60s and 70s Fahrenheit; above treeline, carry a layer regardless of forecast. The mountain bike park is at its best from mid-July through September when trails have dried out fully.
Spring and autumn: May and early June are mud season at lower elevations and still-winter at the resort — neither skiing nor biking is at its best. October closes the bike park and brings the first snow, but the aspens turn gold in late September and the hiking is excellent before the lifts shut down for the season changeover.
Altitude: Keystone's base sits at 9,280 feet. Visitors arriving from sea level should allow a day or two for acclimatisation before hard exertion, and should drink more water than usual. Alcohol hits harder at altitude too.