More than 100,000 people are expected to visited various historic, cultural and unique buildings in neighborhoods across the city and neighboring suburbs on Saturday and Sunday.
"It's as far north as Evanston and Wilmette, as far west as Oak Park, and we're actually in the Austin community on the West Side for the first time this year and as far south as Beverly and Morgan Park," said Eric Rogers, manager of Open House Chicago.
One of the sites, located at 151 North Franklin, is one of the newest office buildings downtown and features a second-floor terrace and rooftop sky garden.
City Hall and the Civic Opera building are also the list.
The Chicago Post Office on West Van Buren is one of four limited sites exclusive to members and volunteers. A handful of visitors will be among the first to see the new renovations to this 1920s-era structure.
"It is one of the most amazing Art Deco buildings in the city of Chicago and of course it's been this hulk over the expressway for so many years that's being brought back to life bit by bit," Rogers said.
Rogers said the event allows people to learn about the city and the people who live here.
The event is free.
For more information and a full list of sites visit: OpenHouseChicago.org