1. In 1874, Lieutenant Colonel George Armstrong Custer and the 7th Cavalry set out on their expedition to the Black Hills of South Dakota, which was then forbidden territory for whites due to an earlier treaty with the Sioux Nation.

2. On May 21, 1949, a massive tornado outbreak swept through the Great Plains, including South Dakota, leaving a path of destruction in its wake. The outbreak produced some of the most violent tornadoes ever recorded, with over 70 people killed and thousands injured.

3. South Dakota became the 40th state of the United States on November 2, 1889, but it wasn't until May 21, 1896, that the state adopted its official flag. The flag features the state seal surrounded by a sunburst and the words "South Dakota" and "The Mount Rushmore State."

4. On May 21, 1927, Charles Lindbergh touched down in Sioux Falls, South Dakota, during his famous trans-Atlantic flight from New York to Paris. Lindbergh made several stops along the way to refuel and rest, and Sioux Falls was one of them.

5. In 1934, the famous outlaw duo Bonnie Parker and Clyde Barrow were killed in a police ambush in Louisiana. Just a few months earlier, on May 21, they had robbed a bank in Platte, South Dakota, making off with over $20,000 in cash and securities. It was one of many robberies committed by the notorious criminals during their crime spree across the Midwest and South.