National Illinois Day is observed every December 7 to recognize Illinois as the 21st state to join the United States. Established by the National Day Calendar as part of its National State Days project, this unofficial holiday celebrates the state's rich history and cultural significance. Illinois, located in the Midwestern United States, is bordered by several states and is known by nicknames such as the Land of Lincoln and the Prairie State. The territory was originally inhabited by various indigenous cultures, notably the Illinois Confederation, before European exploration began with French explorers Jacques Marquette and Louis Jolliet in 1673. The state was officially admitted to the Union on December 3, 1818, although National Illinois Day is celebrated on December 7 as part of a systematic recognition of states in the order of their admission. This day serves to honor the heritage and contributions of Illinois within the broader context of American history.