Billie Joe Armstrong and Billie Eilish. Green Day got things started at the Kia Forum, where they surprised fans with Eilish to help them perform, "Last Night on Earth" from 2009's 21st Century Breakdown.

Two concerts collectively being referred to as FireAid kicked off at the Kia Forum and the Intuit Dome in the Los Angeles city of Inglewood to support relief efforts in the wake of the deadly wildfires that ignited earlier this month in the LA area, claiming 29 lives and causing billions of dollars in damage.

Billie Eilish made a surprise appearance on stage with Green Day on Thursday to kick off one of two star-studded concerts in Los Angeles put together to raise funds for people affected by deadly wildfires in California this month.