1. Battle of the Rosebud (1876): On September 23, 1876, the Battle of the Rosebud was fought between the United States Army and the Sioux Tribe in southeastern Montana. The battle served as a precursor to the infamous Battle of Little Bighorn, which occurred just a week later.

2. Montana Statehood (1889): September 23, 1889, marks the day that Montana officially became the 41st state to join the Union. This event is celebrated as Montana Statehood Day, a holiday in the state.

3. Anaconda Copper Strike (1917): On September 23, 1917, over 10,000 miners went on strike against the Anaconda Copper Mining Company in Butte, Montana. The strike was a pivotal moment in the labor movement in Montana and led to major changes in working conditions and unionization.

4. Fort Peck Dam completed (1940): The Fort Peck Dam, one of the largest earthfill dams in the world, was completed on September 23, 1940. The dam, located on the Missouri River in northeastern Montana, provides hydroelectric power and flood control for the surrounding area.

5. Steve Daines sworn in as U.S. Senator (2015): Finally, on September 23, 2015, Steve Daines was sworn in as the junior United States Senator from Montana. Daines had previously served in the U.S. House of Representatives and was elected to the Senate in 2014.