Killington has two distinct seasons that make it worth visiting. Winter (December through April, and often into May) is ski season — crowded at peak weekends, expensive, and very lively on the access road strip. The best ski conditions are usually February and early March, when snowpack is at its deepest and the natural snow supplements the machines. Avoid the week of President's Day in February if you can: it is the busiest weekend of the year.
Autumn (late September through mid-October) brings the fall foliage — Vermont's most famous natural spectacle. The Green Mountains turn red, orange, and gold, and the Killington area is particularly good because the elevation means the colours peak earlier than in lower valleys. The access road is busy with leaf-peepers but quieter than ski season, and the hiking is at its best. Summer is the quietest season: the resort runs gondola rides and the bike park, but the village feels half-empty between the ski exodus and the autumn arrival.