Poor Curiosity, all alone out there in the vast Martian wastelands with no one to hang out with on its first birthday. So what's an intrepid rover to do? Sing "Happy Birthday" to itself, that's what.
In commemoration of Curiosity's first year on Mars, NASA transmitted instructions to the rover's SAM (Sample Analysis Instrument) module. This device can create audible tones by resonating at varying frequencies. Normally, the resonance helps SAM to filter finely-grounded samples of the Martian surface for analysis.
But in this case, by sending a series of frequencies that corresponded to "Happy Birthday," Curiosity was essentially able to "sing" (or hum, I suppose) the song to itself.
Which is actually kinda cool if you think about it. Though no one was there to hear it — and it would have sounded very faint given the thin Martian air — this famous song was actually performed on the Martian surface.