That’s kind of an awkward pic to use for the purposes of celebrating someone’s birthday, but hey; don’t blame me. Talk to the K-On! movie app.
But yeah, it’s Mio’s birthday. If she graduated from High School in 2010 (which she did, according to the manga), that would mean she started High School in 2007. If she was 15 at that time (which she was, according to the anime), that would mean her year of birth was 1992.
If the current year is 2012 (which it is, according to my calendar), that means Mio turns 20 years old today. Happy Birthday!