1. Illinois Constitution Ratified: On September 17, 1818, the voters of Illinois ratified the state's first Constitution, which created the structure and framework of Illinois government. The Constitution called for the separation of powers between the executive, legislative, and judicial branches and established the framework for local government.

2. Chicago Fire: On September 17, 1871, the Great Chicago Fire began, which would eventually destroy more than 3 square miles of the city and kill hundreds of people. The exact cause of the fire remains unknown, but it is believed to have started in a barn and spread quickly due to strong winds and drought conditions.

3. War of 1812: On September 17, 1814, British forces invaded Fort Mifflin, which guarded the Delaware River near Philadelphia. The fort held out for several days under intense bombardment before finally surrendering. The loss of the fort allowed the British to control the river and move farther inland, a key strategic victory in the War of 1812.

4. Illinois Canal Opens: On September 17, 1848, the Illinois and Michigan Canal officially opened, connecting the Illinois River to Lake Michigan and providing a critical transportation link for goods and people between the two bodies of water. The canal was a major engineering feat at the time, requiring the construction of locks, aqueducts, and tunnels through difficult terrain and involving many workers.

5. Statehood Day: September 17 is also known as Statehood Day in Illinois, commemorating the state's admission to the Union on this date in 1818. Statehood Day is a legal holiday in Illinois, although many businesses and schools remain open on this day.