As an AI model, I cannot guarantee that these facts are the most interesting or comprehensive, but I have compiled 5 notable events that happened on April 1 in Illinois history:

1. The Illinois State Geological Survey was founded on April 1, 1858, making it the first state geological survey in the United States. The survey's main function is to study and manage Illinois' geologic resources and provide services to the state's residents.

2. On April 1, 1893, the World's Columbian Exposition officially opened in Chicago. The Exposition commemorated the 400th anniversary of Christopher Columbus' arrival in the Americas and featured impressive exhibitions and buildings, including the iconic Ferris Wheel.

3. Chicago's O'Hare International Airport opened on April 1, 1955, making it one of the largest and busiest airports in the world. It was named after Lt. Commander Edward O'Hare, a World War II hero and the first naval recipient of the Medal of Honor.

4. The Illinois Tollway system began operating on April 1, 1958, which consisted of three main toll roads: the Tri-State Tollway, Northwest Tollway, and the East-West Tollway. The tollway system is responsible for maintaining and updating Illinois' roadway infrastructure.

5. On April 1, 1975, Illinois became the first state to enact a mandatory seat belt law, requiring all drivers and front-seat passengers to wear a seat belt while operating a vehicle. This law has saved countless lives in the state and continues to be enforced to this day.