Billy Joel sits on a rolling office chair in what is normally the visitors locker room at Madison Square Garden and puts daughter Della Rose on his knee. 

Della has a request. She wants him to play “Don’t Ask Me Why” in that night’s show.

“What will you do when I play that?” Joel asks Della, who will be 3 years old on Aug. 12.

She slides off Joel’s knee and does a little dance, much to the delight of Joel, wife Alexis, and the crew gathered in the room. Joel takes creative director Steve Cohen aside and tell him to move “Don’t Ask Me Why” up in the set to make sure she can see it.

That night, “Don’t Ask Me Why” moved all the way up to the third song in the show. Joel dedicates it to his daughter, telling the crowd, “She’s probably going to be falling asleep soon.”

As the band starts, Joel sits at the piano and shades his eyes from the spotlight to look over to the seats where Della is already dancing with Alexis. Seeing them happy, Joel starts the countdown to signal the band that he is ready to start singing, “Uno! Dos! Uno, dos, tres, cuatro!”

Billy Joel is most definitely a rock star. His American tour this summer is nothing but stadiums — including Fenway Park in Boston, Wrigley Field in Chicago, Citizens Bank Park in Philadelphia. His greatest hits collection has sold more copies than any other album in history, except Michael Jackson’s “Thriller” and the Eagles’ greatest hits. He is a member of the Rock and Roll Hall of Fame and the Songwriters Hall of Fame, and he was honored by the Kennedy Center for the Arts and received The Library of Congress Gershwin Prize for Popular Song.

But these days, the Hicksville, N.Y., native’s life is increasingly about family — both the one that lives with him on Centre Island and the close-knit work family that has helped him put on shows for decades. On Wednesday, Joel is set to headline Madison Square Garden for the 100th time — nearly 40 years after he made his debut there — marking a milestone so high it was considered unthinkable for years.

“I remember my first show at The Garden, that was a milestone,” Joel says. “If someone would have projected that I would do 100 shows there, I would have laughed at them. I’d say you’re being ridiculous.”

Even Jim Dolan, chairman and CEO of The Madison Square Garden Co., said the record seems unbreakable. “Billy Joel is one of the greatest figures in rock and roll history, and he has now accomplished something that might never be equaled — 100 shows in a single venue,” Dolan said in a statement. “This milestone is a testament to the strength of Billy’s music and his incredible connection to his fans — many of whom come from Long Island. All of us at The Madison Square Garden Company look forward to his, and his fans, continuing to make The Garden their home well into the future.”

Joel jokes that he accomplished the stunning feat simply because he didn’t die. (“The secret of success? Just don’t die!” he says, laughing. “The secret of longevity? Stay alive!”)

When pressed for a serious answer, he laughs again. “I don’t have the slightest idea,” says Joel, who began playing monthly at The Garden in 2014, the first (and only) arena-sized residency in music history. “The audiences are great. The venue is great. It’s a world-class venue. To have a residency there is a dream already. I never imagined that anyone could have a residency at The Garden. We’re a franchise. We’re like a sports team. The whole thing has just been one crazy, exhilarating night after another.”

Unlike many superstar headliners these days, Joel keeps most of his show preparations secret. He doesn’t sell VIP concert packages that include access to his sound check. He doesn’t offer meet-and-greet sessions before his shows. He doesn’t even sell tickets to the front row of seats, for that matter. (Those seats are given to excited fans that Joel’s crew finds in the upper-level seats.)

However, Joel granted Newsday access to the backstage preparations of his Madison Square Garden concert on May 23, the 98th Garden show of his career, to see what goes into putting on his show at the arena. More than 1,500 people work at every Joel concert — including everyone from the musicians to the ushers — and most have worked at many, if not all, of the shows in the residency.

Most days start around 8 a.m. with production manager Bobby “Boomer” Thrasher, who handles the setup of the stage and the logistics of putting together everything backstage, meeting with his team. “I wouldn’t call it a science,” says the Ontario native who started with Joel 36 years ago building stages and quickly moved up the ranks. “It’s called a living.”

Thrasher, who has also worked on tours with Bruce Springsteen and Elton John and received the 2018 Parnelli Audio Lifetime Achievement Award, says the residency has given a lot of people a stability that is almost unheard-of among music professionals. “It’s almost like a cab medallion — once you get it, you don’t want to lose it,” says Thrasher, who proudly says his sons Ted, the drum tech, and Lucas, who helps build the stage and works the teleprompter, also work for Joel. “This is where you want to be. This is heaven.”

Most of Joel’s tight-knit crew say the atmosphere comes directly from The Piano Man himself. “With other acts, management handles the hiring and they are often deciding based on the dollar,” Thrasher says. “For us, Billy handles it and he decides based on what’s best for the show. … He brought all of us here. We’re his family. We’re his comfort factor and he’s our comfort factor.”

Joel says he believes the key to keeping his band and crew together is mutual respect. “A lot of these guys have been with me for 40 years or longer,” he says. “ Brian Ruggles has been with me for 50 years. … I respect what they do. I know how hard they work and I hopefully compensate them accordingly. I acknowledge that everyone is as important as everybody else. I know I’m out front, but I still feel like I’m playing in a band.”

Joel has his own dressing room at Madison Square Garden, of course. Actually, it’s a well-appointed suite of rooms. But he prefers to be in the main production room, at the end of Thrasher’s table, not too far from the pizza boxes and chicken wings set up for the band and crew.

That’s usually where the VIP’s come in to meet him before the show. It’s where they’ll gather to sing “Happy Birthday” to Joel’s agent Dennis Arfa and saxophonist Mark Rivera before they take the stage. It’s where creative director Cohen likes to arrive around 10 a.m. on show days, hours before he really needs to be there.

“They’re all my family and we only get to see each other once a month so this is where you want to be on a workday,” Cohen says. “It’s not like other tours where you wait until catering is all set up.”