If you add your client's birthday into their client profile, you'll be reminded on the day of their booking so you can wish them a happy birthday as they walk through the door...
To add a client's birthday, just search for them using client search in the top right of the calendar:
Start typing their name and click on the client to open their profile:
Click on "Edit client details"
Then scroll down to the bottom where you can enter the client's birthday. Don't forget to save changes!