updated 05/21/2012 AT 2:00 PM ET
•originally published 05/21/2012 AT 3:10 PM ET
Mark Zuckerberg may have had an intimate, backyard wedding, but there was at least one famous name on the modest guest list.
Green Day’s Billie Joe Armstrong performed at Zuckerberg’s Palo Alto, Calif., nuptials on Saturday, PEOPLE has learned exclusively.
“They’ve been friends for some time,” a source close to the artist tells PEOPLE of Zuckerberg and Armstrong. “He wasn’t paid. It was a total impromptu wedding present for a friend.”
But it wasn’t the only surprise of the day.