1. In 1972, the first meeting of the Colorado Water Quality Control Commission was held in Denver. This commission was established to monitor, regulate, and protect the quality of Colorado's water as part of the federal Clean Water Act.

2. In 1973, the United States Environmental Protection Agency (EPA) designated the Denver-Boulder area as a non-attainment area for air quality standards. This designation required Colorado to take action to improve air quality and reduce air pollution.

3. On June 23, 1984, John Denver performed a benefit concert at Red Rocks Amphitheatre in Morrison, Colorado, to raise funds for the Windstar Foundation. The Foundation was established by Denver to promote ecological and environmental awareness and activism.

4. In 2012, Colorado suffered one of its worst wildfires in history, the Waldo Canyon Fire. On June 23, this wildfire began in the Pike National Forest near Colorado Springs, eventually destroying 346 homes and killing two people.

5. June 23, 2015, marked the 150th anniversary of the Sand Creek Massacre, a significant event in Colorado's history of conflict between Indigenous nations and white settlers. On this day in 1864, a United States Army cavalry brigade attacked and killed hundreds of peaceful Cheyenne and Arapaho people in southeastern Colorado. The Sand Creek Massacre is remembered as a tragic example of the violence and injustice inflicted upon Indigenous peoples during the westward expansion of the United States.