Winter Park has two distinct seasons and the choice of when to go depends entirely on what you're there for.
Winter (December–March): The ski season runs from mid-November to mid-April in most years. January and February are the best months for skiing — the snowpack is usually at its deepest and the crowds, while significant, are manageable compared to the Christmas–New Year peak. Weekdays are dramatically quieter than weekends; if you can ski Tuesday through Thursday, you will have the mountain largely to yourself.
Summer (June–September): The shoulder months of May and June can still see snow at altitude but the lower valleys clear by late May. July and August are warm, sunny, and excellent for mountain biking, hiking, and fishing. The resort's bike park operates from June through October, weather permitting.
Spring and autumn: April through May sees reduced ski operations and closed mountain bike trails. October and November are quiet and cold — not unpleasant if you want the town without the crowds, but limited in terms of activities.