It is a riddle for the ages: Who left the cake out in the rain? If anyone has insider intel on the matter, it is singer-actor Alex Hairston, who has been immersed in all things Donna Summer in preparation for Broadway in Chicago’s 17th annual summer concert.

The free concert Monday — produced with the city’s Department of Cultural Affairs and Special Events — generally fills Millennium Park to capacity, drawing roughly 11,000 people for a summer harbinger of the coming season in Broadway touring shows.

Songs from “Summer: The Donna Summer Musical” are part of a program that also will include music from the Tony-winning “The Band’s Visit,” Disney’s frosty-freeze ice escapade “Frozen” and the high school-set musical “Mean Girls,” whose book is by Second City vet Tina Fey and inspired by Fey’s 2004 movie of the same name.

Classics from “My Fair Lady,” “Once on This Island” and the seemingly indestructible, chandelier-crashing money machine that is “The Phantom of the Opera” also are on tap, as are selections from contemporary hits “Hamilton” and “Dear Evan Hansen.”

Broadway in Chicago’s 17th annual free summer concert When: 6:15 p.m. Monday Where: Jay Pritzker Pavilion at Millennium Park, 201 E Randolph St. Admission: free Info: Broadwayinchicago.com

But back to the infamous cake so cruelly left to melt in Summer’s disco epic cover of “MacArthur Park.” It’s featured prominently in the “Summer” score. But Chicagoans will have to wait until the show arrives to get the full context.

“Actually, they haven’t told me what I’m going to be singing yet,” Hairston said Wednesday. “But I think it’s going to be either ‘Bad Girls’ or ‘She Works Hard for the Money.’ ”

“Summer” has yet to announce final casting for the touring production. But Hairston has been reading up on the superstar’s life with the intention of being part of it.

Donna Summer is often remembered for her dazzling list of chart-toppers, engineered to make you want to dance or have sex or both. But there’s more to her music than infectious bounce, Hairston said.

“Songs like ‘Bad Girls’ sound at first like just fun, upbeat dance numbers, but Summer used her music to talk about the things she saw,” Hairston said. “ ‘Bad Girls’ is about prostitutes in California. It was about people looking down on them because of what they did for a living. It was about doing what you need to survive. ‘Hard for the Money’ came after Donna went to the bathroom in a club and saw the attendant there sleeping, exhausted.”

Summer isn’t the only outsized personality who’ll be represented Monday. Mary Kate Morrissey was a lovely shade of chartreuse when she came through Chicago last year as Elphaba, the green lead of “Wicked.” On Monday night, Morrissey will perform as Janis Sarkisian, the goth-girl, misfit moral center of “Mean Girls.”

Morrissey sees Elphaba and Janis as kindred souls.

“If Elphaba wasn’t green and was living in the regular world, she would present as Janis,” Morrissey said of the “Mean Girls” outcast.

And instead of defying gravity, she said she’d be belting out “I’d Rather Be Me,” Janis’s 11 o’clock song of celebrating yourself even if the world insists you’re an irredeemable freakazoid.

“One thing I love about this song is that it’s a girl singing to a group of women, and it has nothing to do with men,” Morrissey said. “That doesn’t happen much in musical theater. It’s important to me to tell stories where girls break societal norms to empower themselves. Whether you’re in high school sitting alone at lunch or not, this song is about that.”

For Shereen Ahmed, empowerment is at the core of “My Fair Lady,” the story of Cockney flower-seller Eliza Doolittle and her transformation into a society lady at the hands of imperious (to put it nicely) linguist Henry Higgins. Having performed Eliza in the acclaimed Lincoln Center revival, Ahmed is ready to launch “I Could Have Danced All Night” over the lakefront.

“You can see Eliza taking power from her first entrance,” Ahmed said. “She meets this strange man who claims he can change her status through the power of speech. She takes his money and then shows up at his house saying, ‘OK, I’m here. Let’s go.’ ”

As Disney’s Elsa in “Frozen,” Caroline Bowman has come a long way since she sang in a troll song in the 2013 movie “Frozen.” The trolls are out in the musical version, and 12 new songs are in. Bowman’s charged with “Let It Go,” an insta-classic from the movie that probably everyone under 12 in the audience could karaoke to without supertitles.

“That song is so well-written I don’t have to do much but bring my voice to it,” Bowman said. “We won’t have all the special effects outside that we can do on stage. But I’m going to make it magical. It’ll be so different than singing with the trolls.”