You can send your subscribers automatic birthday wishes or coupons for birthdays by creating a birthday autoresponder. For that, your subscribers must have the birthday parameter in the database. If you don't already have it, read how to import contacts' birthdays.
To create a birthday autoresponder, first go to the "Automation" tab on your homepage (1), and click "New autoresponder" (2) from there.
Then go through all the steps of creating an autoresponder up until the 4th step (if you have not created an autoresponder before, check also the Creating an autoresponder manual).
In the last or 4th step, choose from the delivery date and time the option "On contact's birthday". You also have a possibility to time your email - you can choose how many hours/days/weeks before or after the birthday the email is sent.
Choose the timing you like and click "Save and activate".