A nice way to help build loyalty and make your customer feel special is to send your customer a birthday email on their birthday.
Within the email you could attach a discount code and a message to let them know how special they are to your business.
In the user flow diagram shown above, the customer will receive an email with a discount code and if they arrive to your website you can show them a popup to celebrate their special day.
The goal objective here could be to have the customer engage and use the discount code to encourage a sale.