Bigsky is right--just remind your head waiter of the special event at dinner the first night and he/she will arrange to have a cake brought out that night.
One "warning": if you want the cake brought BEFORE everyone orders (and gets, and eats) other desserts, let your servers know that too; we had several special cakes served on our cruise, and we always saved them for the next night because they arrived after we'd already stuffed ourselves with other goodies! But that won't work for you since you'll be leaving the next day...
Have a great cruise!