From the Colorado Parks & Wildlife news release:
Each year, the agency designates the first weekend in June as the only two-day period that anglers all around the state are not required to have a fishing license. For the rest of the year a fishing license is required for anyone 16 years and older.
A recent survey of resident and non-resident anglers revealed a high level of satisfaction with fishing experiences in Colorado. Anglers said they especially like to pursue the numerous trout species available in state waters. The majority of fish caught in Colorado are stocked by the agency, and each year more than 3 million catchable-sized trout and 14 million trout fingerlings are stocked into the more than 2,000 lakes, ponds and reservoirs around the state.
While fishing licenses are not required during free fishing weekend, all other regulations remain in effect. Anglers should consult the 2013 Colorado Fishing brochure for specific regulations and restrictions for the waters where they'll be fishing.
Fishing licenses can be purchased at any Colorado Parks and Wildlife office or from one of the more than 600 license agents across the state. Licenses can also be purchased online. Anglers may also purchase a fishing license over the phone and receive a temporary authorization number allowing them to fish immediately by calling 800-244-5613.