1. Statehood Day: September 15 is celebrated as Statehood Day in South Dakota since 1889 when the state was admitted to the Union. It is one of the most important dates in the history of South Dakota.

2. Crazy Horse Memorial: The foundation stone for the famous Crazy Horse Memorial was laid on September 15, 1948. The monument is dedicated to the legendary Native American warrior Crazy Horse and is still under construction.

3. Black Hills Expedition: On September 15, 1874, a military expedition led by General George Armstrong Custer explored the Black Hills of South Dakota. This expedition discovered gold in the region, leading to the Black Hills Gold Rush.

4. Mount Rushmore: On September 15, 1927, Gutzon Borglum began work on carving the faces of four American Presidents onto Mount Rushmore. The monument has become one of the most famous landmarks in South Dakota, attracting millions of visitors every year.

5. Deadwood Wild Bill Days: Every year, the town of Deadwood in South Dakota hosts the Wild Bill Days festival over the weekend closest to September 15. This event commemorates the life and accomplishments of Wild Bill Hickok, a famous figure in the history of the Wild West.