Party on, Selena. How did Selena Gomez celebrate her 21st birthday? She released a music video about her birthday, of course! The now-21-year-old songstress unveiled a brand spankin' new video for her track, "Birthday" just in time to coincide with her day of birth, July 22nd. The video features Selena hanging at a wild house party with some energetic folks having a grand old time, dancing, throwing confetti, and doing young people things like fist pumping. Girls night out! Selena Gomez and gal pal Lily Collins were spotted dancing the night away at Hollywood's swanky Roosevelt Hotel Saturday night. The girls made their second stop of the evening after joining a bevy of stars at Julianne Hough's twenties-themed birthday party earlier. Selena Gomez is known for her super cute retro glam looks, but like her BFF Taylor Swift, she knows how to vamp it up! The music video for "Slow Down", recently leaked, shows the singer's sultry, grown-up style as she conquers Paris.