The national holidays can be added with the following steps. Go to Settings > Employees > Public holidays > click on + Import Holidays (top right). A Public holiday setting will pop up and you can select a country and the year. By clicking the Import button, the Public holidays will appear in the schedule.
If you want the Public holidays to be added as an absence, you can best use Bulk actions. To do this you have to go to Employees > select the employees > click on Bulk actions > Add absence. This is where you can add an absence for multiple employees at once.
The following article will provide more information on this subject.