ON MAY 18, Marilyn arrives for rehearsal at Madison Square Garden in a pair of dark sunglasses. She’s tied a scarf over her hair and dressed in a lime-green Pucci blouse, light-colored pants, and white Ferragamo pumps.

Even after practicing her number for weeks, she can never quite shake the inevitable stage fright, especially when she’ll be performing among musical greats Ella Fitzgerald, Peggy Lee, Harry Belafonte, and Maria Callas.

It was Peter Lawford who put the idea in her head to sing “Happy Birthday” to the president. Tomorrow night the fifteen thousand people who’ve paid up to $1,000 for a ticket will see how she’s made it her own.

“Life’s too short to worry about Marilyn Monroe,” Jackie Kennedy says to her sister, Leigh.

The First Lady spends May 19 in Leesburg, Virginia, at the Loudoun Hunt Horse Show, winning a third-place ribbon on her show-jumping horse Ninbrano.

In Marilyn’s dressing room backstage at Madison Square Garden, Mickey Song is styling her hair with a flip curl.

It’s the first time Marilyn is meeting Song, a hairdresser for the Kennedys. He’s cut Jack and Bobby Kennedy’s hair for tonight’s event. “She didn’t want me to work on her, because she didn’t know me,” Song says. “But Bobby convinced her.”

Song is excited to work with the movie star, but he senses that she’s “extremely nervous and uptight.”

That might be because Bobby Kennedy is pacing back and forth outside her dressing room.

“Would you step out for a minute?” the attorney general asks Song.

The hairdresser waits in the corridor. From what he can hear, Marilyn and Kennedy are arguing. Voices are raised. The exchange is growing more intense.

Fifteen minutes later, Kennedy emerges. He looks at Song. “Do you like her?” he demands.

When Song nods his head yes, the attorney general declares, “Well, I think she’s a rude fucking bitch.”

Kennedy stalks off and Song returns to Marilyn.

She’s all disheveled but giggles and says, “Could you help me get myself back together?”

Twelve feet from the stage, the president steps into his private box, signaling the start of the show to be emceed by his brother-in-law Peter Lawford.

Bobby and Ethel Kennedy are sitting nearby. So are Marilyn’s good friend Pat Kennedy Lawford and her publicist, Pat Newcomb. Harry Belafonte sings a moving interpretation of “Michael Row the Boat Ashore.”

Marilyn is scheduled to perform next, but she misses her first cue.

Lawford jokes into the microphone, “Mr. President, on this occasion of your birthday, this young lady is not only pulchritudinous but punctual.”

A spotlight pans to the corner of the stage. It’s empty.

Laughing, Lawford gives her a second cue. “A woman about whom it truly may be said, ‘She needs no introduction.’ Let me just say, ‘Here she is.’”

From the band, a drumroll. Again, the flash of the spotlight to reveal blank space.

The audience laughs along.It’s all part of the show, isn’t it?

Lawford is riffing now but gears up for a final introduction. “In the history of show business, perhaps there has been no one female who has meant so much … who has done more … Mr. President, thelateMarilyn Monroe!”

To thunderous applause, the famously “late” Marilyn finally appears. She crosses the stage in tiny, geisha-like steps, the top of her magnificent dress concealed by a hip-length short-sleeved white ermine coat.

As Lawford helps Marilyn out of her wrap, the audience inhales sharply, and neither Jack nor Bobby can take his eyes off her. No one can.

Marilyn flicks the microphone with her finger. It pops. She’s had nightmares that she’d start to sing but no sound would come out.

“Happy biiiiiirthday to youuuu …”

She performs a breathy rendition of the classic song, with the gentlest hint of innuendo that is smartly judged and perfectly knowing. It’s just as she rehearsed it.

After the briefest of pauses, she launches into a familiar tune. “Thanks for the Memory” may be best known as Bob Hope’s theme song, but tonight the lyrics have been cleverly rearranged. Leo Robin, who wrote “Diamonds Are a Girl’s Best Friend” for Marilyn, has created “Thanks, Mr. President.”