Two Firefly bands to open for Peter Bjorn and John in Philadelphia on Sept. 23.

Swedish pop trio Peter Bjorn and John will whistle their way to the Trocadero Theatre in Philadelphia, Pennsylvania, Sept. 23.

The openers are City of the Sun and Cleopold, two bands that graced Firefly Music Festival in Delaware this past summer.

Peter Bjorn famously broke the mold a decade ago with one of the catchiest songs to ever feature whistling: “Young Folks.”

Now the band is back with their new and seventh album, “Breakin’ Point,” which dropped in June. The project has more whistling for fans. The album also marks the band’s first release in more than five years.

Recently Peter Bjorn recently released a trippy video for their song “Dominoes,” featuring human Dominos.

"Breakin’ Point” was the first release to find the trio working with outside producers. When asked about the decision to venture outside for production help, bassist Björn Yttling said, “You behave better when you have people over for dinner that aren't just your family.”

Over the past five years while recording “Breakin’ Point,” the trio lent their talents to a long list of impressive artists including Lykke Li and Franz Ferdinand, among others.

IF YOU GO

WHEN 7:30 p.m., Aug. 23

WHERE Trocadero Theatre, 1003 Arch St., Philadelphia, Pennsylvania

MORE INFO thetroc.com or call (215) 922-6888