1. Spanish Colonization: On September 15, 1776, Spanish explorer Francisco Atanasio Domínguez and his team arrived in what is now southwestern Colorado. They became the first Europeans to document the area’s unique vegetation, such as scrub oak and juniper.

2. Women's Suffrage: On September 15, 1893, Colorado became the first state in the U.S. to allow women to vote in a state election by constitutional amendment. This preceded the 19th Amendment, which granted women throughout the country the right to vote in 1920.

3. Colorado Statehood: On September 15, 1876, President Ulysses S. Grant officially declared Colorado as the 38th U.S. state. The Colorado State Capitol building in Denver was completed in 1894 as a symbol of the state's prosperity and commemoration of its statehood.

4. Natural Disasters: On September 15, 2013, record-breaking rainfall led to severe flash flooding in multiple Colorado counties, resulting in at least eight fatalities and millions of dollars in damages.

5. Sports History: On September 15, 1975, the Denver Broncos faced off against the Oakland Raiders in a Monday Night Football game that would go down in history as one of the most exciting. Quarterback Ken Stabler led the Raiders to a 23-20 win, now known as the "Sea of Hands" game after a dramatic catch by Raiders running back Clarence Davis with only seconds remaining.