From the OpenHouseChicago.org website:

The Chicago Architecture Center’s Open House Chicago (OHC) is the city’s annual architecture festival. For one weekend in October you can explore Chicago’s most iconic and unique architectural treasures. From mansions to sacred spaces, theaters to private clubs, hotels to secret rooms—OHC gives visitors a behind-the-scenes look at many of the city’s great spaces that are rarely, if ever, open to the public.

OHC is a citywide event that includes more than 350 sites located in more than 38 neighborhoods. Sites reflect the cultural diversity and history of Chicago, as well as the unique character of each community. Locations include private clubs, residential spaces, offices, hotels, theaters, design/architecture studios, schools and places of worship as well as manufacturing, cultural and government facilities. This diverse selection of sites allows visitors to plan an itinerary according to their own specific interests. Browse the sites that have participated in OHC since 2015.

To read more: https://openhousechicago.org/