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". 

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