The singer and her agent beau Trawick, 37, started the celebration a day early Tuesday night at L.A.'s Mondrian hotel, where Spears also enjoyed spa services, a source tells PEOPLE.
That night, the couple went to the Sunset strip, visiting both the Saddleback Ranch restaurant and Chateau Marmont. The next day, they shopped at West Hollywood's Fred Segal boutique before reuniting with the pop star's sons Preston, 3, and Jayden, 2, at her home in Calabasas.
"Britney looked so happy," says an eyewitness. "She seemed very excited to spend her morning with Jason."
Spears was also thrilled to get to be with her two boys on her B-day, says another source. "Britney is very happy to spend the day with Preston and Jayden. The kids know it's her birthday … [They] are very excited about Britney's cake too."
Britney's Ex Celebrates TooWhile Spears hung with her sons at home, ex-husband Kevin Federline celebrated his girlfriend Victoria Prince's birthday – which happens to fall on the same day as Spears's.
Federline, 31, and Prince, 27, had a late lunch at Mexican restaurant Casa Vega in Sherman Oaks, Calif. "She looked very pretty when she arrived for her bash," says an observer.
Federline, who sources says has lost about 30 lbs. since joining the cast of VH1's upcoming Celebrity Fit Club, "joked how it's about time he gets some proper food," according to an eyewitness.