Look here for all the days you can get in free:
http://www.msichicago.org/visi.....free-days/
Also I read that you can get a free pass to any museum in Chicago from a Chicago Branch Library.
It's not easy to get them though cause more often than not, they are checked out already and the libraries have limited passes.
I cannot verify this info since I don't live in IL. But those of you that do, may want to call a Chicago library and see if this is for real.