Obama also tweeted on Wednesday that Michelle is "the love of my life and my best friend."
Twenty years ago today, I married the love of my life and my best friend. Happy anniversary, Michelle. -bo— Barack Obama (@BarackObama) October 3, 2012
The first lady was in Reno, Nev., earlier in the day, but will be at the University of Denver on Wednesday night to watch the debate.
Obama's campaign posted a wedding photo of the couple on the president's Twitter feed.
Updated at 3:20 p.m.