Gwen Stefani celebrated the big 4-0 on Saturday with an afternoon birthday bash at the exclusive Italian restaurant Cecconi's in West Hollywood.

The singer, whose actual birthday is Oct. 3, was joined by her hubby Gavin Rossdale, their two children – Kingston, 3, and Zuma, 13 months – and an intimate group of friends, including Kate Beckinsale and her husband Len Wiseman.

The celebration took place in the restaurant's romantic and private Butterfly Room, which features a $1 million heart shaped Damien Hirst painting and a crystal chandelier. "The Butterfly Room was specially decorated for Gwen's celebration with happy birthday balloons and roses," says a restaurant source.

The group enjoyed a family style lunch with several Italian appetizers, pasta dishes, salad, champagne and wine.

"Gwen looked amazing and so much younger than 40. She was in a great mood and seemed very excited to celebrate her birthday with her husband, children and friends," says the source. "Gwen and Kate seem very close and the girls shared a long chat and many laughs. Gavin and a nanny took care of the boys so Gwen could just relax and enjoy her celebration."

After a long Italian feast, Gwen was surprised with a big birthday cake decorated with candles and sparklers. "With her oldest son helping her to blow out the candles, Gwen made a wish and she looked very happy," the source adds.

Stefani's birthday celebration actually started the night before when she and Rossdale enjoyed a night out without their kids during a party at the Oceana Hotel in Santa Monica. "Gwen and Gavin spent a great night with friends and looked very happy together," says a hotel source. "When the clock struck midnight, Gwen and Gavin toasted to Gwen's big birthday."