Credit: Allyson Magda Photography

Mark Zuckerberg and Priscilla Chan are good at keeping secrets.

On Saturday, the couple invited less than 100 friends and family members to celebrate Chan's graduation from from medical school at the University of California, San Francisco, a guest authorized to speak for the couple tells Us Weekly. What they didn't know, however, was that Facebook founder and CEO Zuckerberg, 28, and his Harvard sweetheart Chan, 27, would also be tying the knot at their Palo Alto, Calif. home.

The low-key ceremony occurred in their backyard. The couple -- who have been together nine years -- had been planning the ceremony for the past five months. Zuckerberg wore a suit and tie, and Chan wore a $4,700 embroidered Claire Pettibone dress featuring matte sequins and ivory cut-outs with a sheer back.

Zuckerberg designed a "simple ruby ring" for Chan himself; she did not see it until the wedding.

Guests ate family style from the couple's favorite local eateries, Palo Alto Sol and Fuki Sushi. For dessert, attendees ate burdick chocolate "mice," which the couple had on their first date.

It's been a banner week for the couple, as Chan graduated on Monday, the same day Zuckerberg turned 28. Four days later, Facebook went public; the move brought Zuckerberg's wealth to an astounding $19 billion.