ET teamed up with KIIS-FM's Jingle Ball to party with the biggest celebrities in music entertainment -- including Derek Hough, who opens up about why his sister Julianne Hough may possibly be leaving "Dancing with the Stars."

"I don't think anyone is done dancing … it's always a part of us," Derek tells ET's Kevin Frazier about Julianne leaving the show to pursue country music. "Sometimes you just want to pursue something and try to build something."

Kevin also caught up with Rihanna, Chris Brown, David Archuleta, Katy Perry, Jesse McCartney, Christina Milian and The Pussycat Dolls. Kevin even had a spontaneous visit from KIIS-FM morning show host himself, Ryan Seacrest.