1. The Sand Creek Massacre occurred on December 16, 1864. In what is now southeastern Colorado, a United States Army force attacked and killed over 150 Cheyenne and Arapaho people, mostly women, children, and elderly, who had been camped under the protection of the American flag. The massacre is now widely considered a shameful episode in the history of the American West's settlement.

2. On December 16, 1876, Colorado achieved statehood. The centennial anniversary of this event was celebrated in 1976 with a series of festivities and commemorative coins and stamps.

3. The Colorado Fuel and Iron Company, a major player in the state's coal mining and steel industries, was incorporated on December 16, 1892. The company went through several mergers and acquisitions over the ensuing decades and was ultimately bought out by Bethlehem Steel in 1993.

4. On December 16, 1937, a devastating explosion occurred at the Columbine Mine in northern Colorado, killing 34 miners and injuring many others. The disaster led to increased safety regulations and unionization efforts among Colorado's mining workforce.

5. In 1955, the Eisenhower Tunnel, one of the world's highest vehicular tunnels, was completed on December 16. Located at an elevation of over 11,000 feet (3,352 meters) in the Rocky Mountains, the tunnel spans nearly 1.7 miles (2.7 kilometers) and allows traffic to pass through the Continental Divide more easily, especially during the winter months.