Denver's reputation for sunshine — 300 days a year — is real, and the weather is more often pleasant than not. The trade-off is altitude: at 5,280 feet, sun is fierce, dehydration is faster than you expect, and the air at night chills quickly even in midsummer.
Late spring (May–June) is arguably the best time: long warm days, wildflowers in the foothills, baseball season in full swing, and the high country opening up week by week as the snow melts off.
Summer (June–August) brings reliable highs in the 80s F and the city's outdoor calendar — Civic Center Eats, free movies in Skyline Park, jazz at City Park, baseball at Coors Field. Afternoon thunderstorms over the mountains are nearly daily; plan hikes for the morning.
Autumn (September–October) is the second great window. The aspens turn gold in late September in the high country; daytime temperatures stay mild, and the crowds thin out. Football season fills Empower Field.
Winter (November–March) is for the mountains. Denver itself gets less snow than people imagine, and downtown remains walkable most days. The big draw is the proximity to the ski resorts — Loveland and A-Basin both within 90 minutes, Vail and Breckenridge within two hours.