1. Historic Gold Discovery: On August 21, 1876, gold was discovered in the Black Hills of South Dakota. This led to a gold rush and the establishment of several mining towns. This event had a significant impact on the development of South Dakota, as well as the history of the American West.

2. First-Ever Sturgis Motorcycle Rally: The first Sturgis Motorcycle Rally was held on August 21, 1938, in Sturgis, South Dakota. It was initially a small gathering of motorcycle enthusiasts, but it has since grown into one of the largest motorcycle events in the world. Every August, thousands of bikers from around the world make their way to South Dakota to participate in the rally.

3. Lewis and Clark Expedition: On August 21, 1804, the Lewis and Clark Expedition passed through what is now known as South Dakota on their journey to explore the newly acquired Louisiana Purchase. The expedition is known for its exploration of the American West, as well as its interactions with Native American tribes.

4. Homestake Mine Closes: On August 21, 2002, the Homestake Gold Mine in Lead, South Dakota, closed. The mine had been in operation for over 125 years and was one of the largest and deepest gold mines in the country. Its closure had a significant impact on the local economy and the mining industry in South Dakota.

5. Fort Pierre Chouteau: On August 21, 1832, Pierre Chouteau Jr. established Fort Pierre Chouteau on the Missouri River in what is now Stanley County, South Dakota. The fort was a trading post for fur traders and played an important role in the fur trade of the American West. Today, the fort is a state historic site and is open to visitors.