1. Statehood Day: On September 28, 1889, South Dakota officially became the 40th state to join the United States. This date is now known as Statehood Day and is celebrated annually in South Dakota.

2. Mount Rushmore National Memorial Completed: On September 28, 1941, the Mount Rushmore National Memorial in the Black Hills was completed. The famous landmark features the faces of four U.S. presidents: George Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincoln.

3. Homestake Gold Mine: On September 28, 1959, the Homestake Gold Mine in Lead, South Dakota broke a world record for being the deepest mine in the world, reaching a depth of over 8,000 feet. The mine operated for over 125 years, producing over 40 million ounces of gold.

4. Sioux Nation Treaty: On September 28, 1868, the Treaty of Fort Laramie was signed between the United States and the Sioux Nation. The treaty established the Great Sioux Reservation, which covered most of present-day South Dakota west of the Missouri River. However, the U.S. government did not uphold the treaty, leading to decades of conflict between the Native Americans and the settlers.

5. World's Largest Corn Palace: On September 28, 1892, the World's Largest Corn Palace was dedicated in Mitchell, South Dakota. The palace, which is still a popular tourist attraction, is decorated each year with murals made of corn and other grains.