"Billie got rushed to the hospital last night due to illness, and the doctors don't think it's a good idea for him to play today," bassist Mike Dirnt said in a video on the band's website. "This sucks. We really, really looking forward to playing."
Armstrong's bandmates didn't elaborate on what prompted his trip to the hospital, but drummer Tre Cool said in the video: "We're beyond devastated. We're as sad as you guys are. Billie is extremely upset about it."
Italian Green Day fans haven't had much luck seeing the band live in recent years. In 2010, they were forced to cancel a show in Venice due to a cloudburst.
We wish Armstrong a speedy recovery!