In the Cruise Personalizer you can fill in Special Occasion portion, letting them know you are celebrating a special occasion. Princess will tape balloons to your door and deliver a card in your cabin, which you can take to take to the maitre'd at dinner. They will bring you a very small cake for dessert. Sometimes they don't get the message, so If you don't get a card by the time your special day arrives, just tell the maitre'd when you arrive for dinner or tell your steward.