What if a customer asks to use an expired birthday card

You still gain regardless of when they redeem their card

While it is no doubt advantageous to entice customers to come in on or near their birthday, fact is, you still gain regardless of when they redeem their card.  Some customers will ask if their card can be used after the expiration date (usually because they had prior plans).  Whether they call or just show up, you handle the situation by letting them know that you are pleased to make an exception and extend the expiration date, which instantly makes them feel as if you are doing them a special favor.  Over the phone, an employee who takes the call should tell the customer to use their (the employee’s) name if anyone questions the extension.  In practicality, it should never be an issue since you don’t need to check the expiration date anyway.  You make the sale and leave the customer with a positive experience all the way around.