Green Day invited a 10-year-old fan on-stage to perform with them during their recent headline show at Outside Lands festival.The rock trio topped the bill at the three-day San Francisco event last Saturday (August 6), delivering a career-spanning 22-track set on the Lands End main stage.At one point in the gig, frontman Billie Joe Armstrong singled out a young crowd member called Montgomery – who the singer nicknamed Monty – and asked him to join Green Day for a special performance.“Do you really know how to play?” Armstrong questioned Montgomery. “Alright, kid – get your ass up here!”Today (August 11), Green Day shared footage of the moment on social media.