National South Dakota Day is observed every April 26 to recognize South Dakota as the 40th state to join the United States. The day was established by National Day Calendar as part of its National State Days project, which aims to honor the history and culture of each U.S. state in the order of their admission to the Union. South Dakota, known as the Mount Rushmore State, is celebrated for its geographical significance in the Great Plains and its rich Native American heritage, particularly linked to the Sioux people. The state was part of the Dakota Territory, created in 1861, and became a separate state alongside North Dakota on November 2, 1889. The holiday acknowledges the state's historical journey from early Native American habitation to European exploration and eventual statehood.