First, create your birthday template under Volunteers > Communication (Email).
There is a link just above the dialogue box to "create and manage templates." Once you've named and saved a "Happy Birthday" template message, go to Account > Settings and scroll down to Automated Messages.
You’ll be able to select your birthday template from the drop-down. This will now send the happy birthday message you've created to each volunteer on his/her birthday.
Please note: for this feature to work, the volunteer's birthdate information must be added as a field in the volunteer information form (Events > Your Events > Volunteer Information). Be sure to label the field with the field designation "birthdate".