1. On April 20, 1972, the Rapid City Flood occurred, causing massive damage and claiming the lives of 238 people. It remains the deadliest natural disaster in South Dakota history.

2. In 1910, on April 20, the town of Pierre was officially designated as the state capital of South Dakota. This decision was made after decades of debate and voting by the state's citizens and lawmakers.

3. On April 20, 1945, the Battle of Box Canyon took place in western South Dakota. It was a conflict between local law enforcement and a group of outlaws who had been using the canyon as a hideout. The outlaws were eventually defeated and several were killed or captured.

4. In 1978, on April 20, construction began on the Crazy Horse Memorial in the Black Hills of South Dakota. The massive sculpture, which depicts the Lakota warrior Crazy Horse riding a horse and pointing into the distance, is still being worked on today and is expected to be one of the largest sculptures in the world when completed.

5. On April 20, 1987, a series of tornadoes swept across South Dakota, causing significant damage and claiming the lives of six people. The tornadoes were part of a larger outbreak in the Great Plains region, which affected several states and caused widespread destruction.