A source tells ET that, on Wednesday, the singer had a "super cute, fun and low-key pool party" at her house to celebrate with her girlfriends.
Simpson's husband "Eric was there and took on hosting duties," the source says of the party, where the guests dined on BBQ, Simpson's favorite.
Additionally, the source reveals that all three of her children -- Birdie, 3 months, Ace, 6, and Maxwell, 7 -- attended the bash.
"Maxi and Ace even chose to stay home from camp to celebrate their mom," the source says, adding that, "Maxi was swimming the whole time."
Simpson took to Instagram on Wednesday to share photos from the soiree, where she wore a leopard-print bikini and matching swimsuit coverup. Simpson, who wore her hair half-up, half-down, also donned festive sunglasses, hoop earrings and a simple necklace.
"39 is all mine!!!" Simpson captioned a solo shot of herself gazing at the camera.
The party came the same day that Simpson announced her forthcoming memoir, which is set to debut in February 2020.
"I have been waiting for so long to share some exciting news...I will be releasing my first book on February 4th with @harpercollinsus," she captioned a photo of herself smiling at the publisher's office. "I have been working so hard on it - I have opened my heart up in a way I never have before and now I can’t wait to share it with the world. 💚"