If you would like to create a birthday reminder for active employees, go to Settings >> Reminders. Tick the box for Birthday Reminders On, which is located at the bottom-left of this screen. Select how many days notice you wish to have, then click on Create Birthday Reminders For Active Employees. This will add a birthday reminder for all active employees.
You can edit each reminder by clicking on the pencil icon and saving those changes; you can also remove reminders for individual employees by clicking the red X in line with their reminder if you/they do not wish to have a birthday reminder for them. You can remove all reminders quickly by unticking Birthday Reminders On at the bottom-left of the screen.
When entering into Lightning Payroll at the time each reminder is set, you will be notified of the birthday reminder by way of a pop up.