If it must be narrowed down to a particular time of year, summer is peak season in Boulder. Then again, so is every weekend that the University of Colorado football team has a home game. Same goes for such important events as the Conference on World Affairs (early April), the Bolder Boulder 10K Road Race (Memorial Day Weekend), the three CU graduations every year and the weeks when students are moving in and out, often with parental assistance. May and June weekends are popular for weddings and can get pretty peak-ish too. That said, summer weather here is beautiful—warm days and cool nights— and it's the best time to visit Boulder.
Mid-winter is as off-season as Boulder ever gets. Lodging rates might drop midweek and sometimes on weekends. Then again, often they don’t.
With so much visitation to the University of Colorado, the National Center for Atmospheric Research, the National Institute of Standards and Technology and assorted high-tech, research and sporting-goods businesses, including sexy start-ups, business travelers occupy hotel rooms much of the year.