Hi I just recieved ours today as well . From my understanding it is birthdays that happen while you are there. If yours is during that time by all means put it down, NOT TACKY AT ALL!!
Beware thought about fudgeing dates, they are putting in a new reservation system according to another board that will track your birthdays, where you stay , where your PS's are and such..
basically there computer will have a file on your stay.
I believe this because yesterday i recieve a info packet stating that i am staying in the Poly, i have shown interest in dinner shows (i reserved the luau) and other things that i didn't tell CRO when i booked the room only.
Hope you have a great trip
Sharon