1. The Battle of the Big Hole: On December 21, 1876, the Nez Perce War ended with a violent confrontation between the U.S. Army and the Nez Perce tribe in what is now known as the Battle of the Big Hole. The battle resulted in the deaths of approximately 60 Nez Perce people, including women and children, as well as 30 U.S. soldiers.

2. Montana becomes a state: December 21, 1889 was a significant day for Montana as it officially became the 41st state of the United States of America. This followed decades of territorial status and a long process of political and economic development.

3. First winter ascent of Granite Peak: On December 21, 1923, Elers Koch led the first successful winter ascent of Granite Peak, the highest peak in Montana. Granite Peak stands at an elevation of 12,807 feet (3,904 m) and is located in the Beartooth Mountains.

4. The Montana State Capitol Building opens: December 21, 1902, marked the grand opening of the Montana State Capitol Building in Helena. Designed by architect Charles Emlen Bell, the Capitol Building is an iconic landmark in Montana and features stunning neoclassical architecture.

5. World War II impact on Montana: On December 21, 1941, just days after the attack on Pearl Harbor, the Montana National Guard was mobilized for World War II. The war had a significant impact on Montana, with residents contributing in various ways such as joining the military, working in the defense industry, and supporting war efforts through fundraising and volunteering.