National Illinois Day Date in the current year: December 7, 2025

Located in the Midwestern United States, Illinois is bordered by Indiana, Iowa, Kentucky, Missouri, and Wisconsin, and separated from Michigan by Lake Michigan. Its borders are largely defined by the Wabash, Ohio, and Mississippi rivers. Illinois’ nicknames include the Land of Lincoln, the Prairie State, and the Inland Empire State.

Before European explorers arrived, the territory of present-day Illinois was inhabited by several successive indigenous cultures. At the time of European contact, the region was dominated by the Illinois Confederation, a loosely organized group of about a dozen tribes, including the Cahokia, Kaskaskia, Michigamea, Peoria, Tamaroa, and others.

The first Europeans to explore the area were the French explorers Father Jacques Marquette and Louis Jolliet in 1673. They came across the Grand Village of the Illinois, a large Native American village in the Illinois River valley. Two years later, Marquette returned and founded a Jesuit mission there.

Over the next few decades, the French established more missions, forts, and trade posts in the region they called the Illinois Country (Pays de Illinois) or Upper Louisiana (Haute-Louisiane). The vast Illinois Country territory included portions of present-day Illinois, Missouri, Indiana, Iowa, Michigan, Wisconsin, Ohio, and Kentucky. It was administered as part of New France, specifically French Louisiana, until 1763.

After losing the Seven Years’ War, France ceded much of the Illinois Country to Great Britain under the 1763 Treaty of Paris. In 1774, this territory became part of the British Province of Quebec.

During the American Revolution, the Illinois campaign was part of the war’s western theater. As a result, the Commonwealth of Virginia claimed the portion of the Illinois Country west of the Ohio River and east of the Mississippi River. These lands were organized as Illinois County, encompassing most of present-day Illinois and parts of present-day Indiana.

In 1784, Virginia formally ceded the land to the United States government. It became part of the Northwest Territory in 1787 and then the Indiana Territory in 1800 following Ohio’s split. In 1809, the United States Congress established the Illinois Territory, which included present-day Illinois, Wisconsin, and parts of Michigan and Minnesota. On December 3, 1818, Illinois was admitted to the Union as the 21st state, and the remainder of the former Illinois Territory was incorporated into the Michigan Territory.

National Illinois Day is not celebrated on the anniversary of the state’s admission because it is part of the National State Days project launched by National Day Calendar in 2017. In this project, states are celebrated in the order of their admission, starting with National Delaware Day on July 13. In this cycle, National Illinois Day falls on December 7, which is fairly close to the state’s actual admission anniversary.