From PinataIsland.info, the Viva Piñata wiki
On your birthday (or any time that the internal clock in the Nintendo DS matches the birthday set in it's user information) you are presented with an announcement, "Pinata Central wishes you a very Happy Birthday. We would like to celebrate the occasion by sending you a tasty cake, stuffed full of candy. Just bash the cake with your shovel to release the treats inside!"
When bashed, the cake releases romance sweets, happy sweets, and a pinata egg.