!! This trophy is missable, so be warned in advance. !!
In Stinky's Diner, talk to the Bug (from Season 1, not available on PS3). He'll tell you that he is hiding out from his family due to his birthday. Once you exhaust all common conversation topics with him, exit the conversation tree and then speak with him once more. The birthday mariachi will appear and play a ditty for the birthday bug and give you this trophy.
This trophy is unobtainable during your playthrough if you've revealed the demon of the North Pole and are requiring help from the Three Christmas Ghosts.