Here you go, I have found the terms & conditions for the Birthday Fun Card
Birthday Fun Card
4. No portion of the Birthday Fun Card balances will be redeemed for cash.
Suggests that the card can be used against multiple purchases, it appears to be like a swipe card with an amount of money loaded onto it that you can spend as and when you like on the day.
I am sure I read somewhere that the card is only valid on the day it is issued.