You can use the Tasks app to create reminders on the day or a fixed number of days before the birthday of a contact, e.g. for a sales rep to send a congratulations to a client.
First create a Task Template named e.g. “Birthday” for a recurring task, i.e. with the Frequency set to ‘Every year’. For convenience, you can create a task list named “Birthdays” to store all birthday reminders, and set this list in the template.
To set a birthday reminder, goto the Contact, add a new “Birthday” task from the template list, and set the due date to the next Birthday occurrence. If you prefer to set the due date to a few days before, you can specify the actual birth date in the task name.
On the due date, you will receive a reminder by email and a notification on your Android phone. After you mark the task as complete, a new birthday reminder task will automatically be created for the following year.