Jen Flies Solo at 40

He's Just Not That Into You opens five days before Jennifer Aniston’s 40th birthday, and the timing couldn’t be more ironic. Jen, one of the romantic comedy’s stars, has decided she’s just not that into John Mayer and has given the singer his walking papers for the second time, sources tell OK!.

That means she’ll be on her own when she celebrates her big 4-0 on Feb. 11 with Courteney Cox and David Arquette, Sheryl Crow, Laura Dern and Ben Harper and Tobey Maguire and Jennifer Meyer — the pals she vacationed with in Los Cabos, Mexico, over the New Year’s holiday.

Though John, 31, was in Cabo too, he stayed in a separate villa with his two brothers and has not been spotted with Jen since returning across the border. In fact, John was in a bleak mood as he entered his NYC apartment on Jan. 25, and when OK! asked about the status of his romance with Jen, the usually chatty singer remained mum.

But even without a man, Jen is excited about her milestone. “I’m more at peace in my mind and my body,” she says.

She’ll no doubt look stunning at her birthday bonanza, expected to be held at her 10,000-square-foot Beverly Hills mansion, which she bought more than two years ago and is still renovating. “There are workmen here well into the early evening,” a neighbor tells OK!. “She wants everything absolutely perfect before her party.”

“The party will be joyous, even without John,” the insider tells OK!. “For Jen, turning 40 and moving into her home are signs that she’s gone on to a new stage in her life. This is a house that has no Brad Pitt footprints, and she couldn’t be happier.”

For more on Jen's big bash, pick up the new issue of OK!, on sale everywhere Thursday!