Picture this ... you're celebrating your wedding day ... it's beautiful ... and just when you think it can't get any better -- KENNY FREAKIN' LOGGINS takes the stage to perform an impromptu version of "Footloose."
Well, it happened ... and we got the video evidence to prove it.
It all went down a few weeks ago in Michigan -- sources tell TMZ, Loggins was invited to the wedding as a guest because he's friends with the bride's dad.
Kenny wasn't there to perform -- but we're told the bride's father made a special request for Kenny to kick off the party with a couple famous tunes ... and Kenny obliged.
Someone should write this into a romantic comedy. Too late, it's our idea.