You can send your subscribers automatic birthday wishes or coupons for birthdays by creating a birthday automation. 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 automation workflow, first go to the "Automations" tab on your homepage (1), and click "New automation" (2).
After clicking on new automation, a workflow builder will open. You can name the workflow (1) and click on "+" icon to choose a trigger. For birthday automation you should choose "schedule" as the trigger (2).
The workflow is triggered at a time specified in subscriber's data (birthday field). You can also choose the offset, if the trigger starts before or after the date specified in the bithday field.
For the next step click on the "+" icon (1) to choose an action. To send a birthday letter, the action should be "send message" (2).
Fill out the information about the subject, from address, from name and a template (1). If everything is ready, save the automation (2).
Under the automations tab you can activate the birthday automation workflow, by clicking on start.