Denver [1] is a draw for tourists year-round, but is generally more crowded in the summer when there are good road conditions for driving into the mountains. This is high season, when hotel rates tend to be at their peak.
Spring is the shoulder season, when flowers are in bloom in the city, but in towns close to the Continental Divide it’s still snowing.
Fall is lovely in Denver, especially in the foothills as the trees change from green to gold and light up the hillsides.
The low season in the city is in winter, when hotels and attractions offer many discounts.
Links:
[1] http://www.moon.com/destinations/denver