1. In 1889, South Dakota was admitted as the 40th state of the United States on September 20. This day is celebrated as Statehood Day in South Dakota, which marks an important milestone in the state's history.

2. In 1922, the world's largest grain elevator was completed in Superior, South Dakota. The elevator was capable of storing over 1 million bushels of grain and played a vital role in the state's agricultural industry.

3. On September 20, 1947, the United States Air Force began its first experimental rocket-powered aircraft flights at the Sioux Falls Municipal Airport. The tests were conducted to evaluate the potential for using rocket engines to power fighter planes, marking an important milestone in aviation history.

4. The Black Hills Powwow, one of the largest annual gatherings of Native American tribes in the United States, takes place during the third weekend of September, often including September 20. The powwow celebrates the traditions and cultures of the Lakota, Nakota, and Dakota people, who have called the Black Hills home for thousands of years.

5. In 1979, a major flood swept through Rapid City, South Dakota, killing 238 people and causing extensive damage to the community. The flood was caused by heavy rainfall and led to changes in the city's infrastructure to ensure better protection against future flooding. September 20 marks a somber anniversary of this tragic event in South Dakota's history.