Unless Michael's transitioning, I'm guessing the wedding of Sarek's son - that Picard once mentioned attending - was Spock's.
Cut dialogue from that scene:
RIKER: I remember studying his career in school. The treaty of Alpha Cygnus Nine, the Coridan admission to the Federation, the Klingon Alliance.
PICARD: I met him once, many years ago, very briefly at his son's wedding.
RIKER WHISPERS SOMETHING IN PICARD'S EAR.
PICARD: What?!! But with the name Michael, I thought....Oh, bugger all!