I found this website that list the free date in 2019. https://chicago.thelocaltourist.com/free-days-shedd-aquarium-chicago/
Free days are posted in the website
There are also groups of people who get in free every day- only with I'd: Teachers from Illinois, Indiana, Michigan and Wisconsin (need to register online) Active-duty U.S. military personnel Blue Star Families Program Chicago police officers and firefighters Plus a lot more with discounted tickets: https://www.sheddaquarium.org/plan-a-visit/Advance-Ticket-Options-Tickets/
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