To do this, you need to update the user's entitlement either individually or in bulk, then:
Go to each employee's planner settings.
Tick the Entitlement Includes Public Holiday? option.
To apply this to all new users going forward you need to adjust the system settings:
Click Access Button
then click HR Admin.
Go to Settings and select Holidays and Absences.
Tick the option Does the Total Holiday Entitlement include Public Holidays?.