Bruce Springsteen has welcomed several special guests while touring this summer, but you'd be hard pressed to find anyone with a more "kid in a candy shop" reaction to it than Mumford and Sons singer Marcus Mumford.

The vocalist, speaking with the Sunday Times (via NME), recalled the summer performance at the Pinkpop Festival, explaining, "It was the best f---ing day of my life - unbelievable." Mumford went on to add, "The Boss had watched our show, which I didn't know until the penultimate song, when I turned around and saw him in his Ray-Bans, nodding."

Mumford says they repaid the attention by watching Springsteen's set from the monitor desk, but when the song 'Hungry Heart' came on they got the nod from the singer to join them onstage. Mumford recalls, "He grabbed me round the neck and shoved a microphone in my face."

The vocalist says, "Have I watched the YouTube footage? Absolutely, from every single angle. I never watch our band, but I'll never stop watching that." Mumford and Sons have been on tour of late building the buzz for their latest disc, 'Babel,' which is due Sept. 25.

Springsteen, meanwhile, has one of the year's top tours and continues to invite his fellow musicians to take part. Over the weekend, Tom Morello and Eddie Vedder joined him at Chicago's Wrigley Field, and earlier this summer Paul McCartney and the Roots spent stage time with "The Boss" in London and Denmark.