Here is a nice summary from the Chicago Department of Cultural Affairs and Special Events August 23, 2018 email newsletter.
Museum of Science and Industry
Free Admission for Illinois residents
September 4-6, 10-13, 17-20, 24-27, 9:30am-4pm
Museum of Science and Industry, 5700 S. Lake Shore Dr.
Visit the largest science museum in the Western Hemisphere during one of the Illinois Free Days (Museum Entry is free for all Illinois residents who show valid proof of residency!) Housed in the only remaining building from the 1893 World's Fair, Museum of Science and Industry is a Chicago must-see.
John G. Shedd Aquarium
Illinois Resident Discount/Free General Admission Days
Tuesday, September 4 - Sunday, September 30, 9am-6pm
Shedd Aquarium, 1200 S. Lake Shore Dr.
Illinois residents enjoy FREE admission to Shedd Aquarium on select free days throughout the year!
Illinois Resident Discount Days
Sunday September 9-Sunday, September 16, 9:30am-4pm
Museum Campus, 1300 S. Lake Shore Dr.
Illinois residents can enjoy complimentary general admission
on Illinois Resident Discount Days. A valid ID with an Illinois address is required for these special admission prices.
Just for Illinois residents: Spend a day at the Museum-for free!
Saturday, September 15, Friday, September 17 and Sunday, September 30
9am-5pm, with last admission at 4pm
Field Museum, 1400 S. Lake Shore Dr.
On Free Admission Days, Basic admission is free for Illinois residents with proof of residency.
Free Admission Days are the perfect opportunity to explore our general admission exhibitions with your family or a group of science-loving friends.