Go to the section "Absences" and click on "Special Days". Now click on "create new day".
A new window will open where you can create the special day. You can make the following settings:
In order to show public holidays in the weekly schedule you can import them in this section. Click on "import public holidays":
A new window will open, where you can chose country, region and year. To finish the import of the public holidays into the weekly schedule simply click on "import public holidays".