You can add an absence in two ways.

The first way to do it is via the agenda. Go to your calendar/agenda and select "create appointment" button, which is at the top right corner of your page.

In the next window, select "absences" and add a description. Select the date and time of the absence. You can apply te absence to the entire company, employee, room tool or location (in case of a Multi-Location set up in your account). See screenshot:

The second way to do it is via settings > company > settings.

Once you've selected "absences", you'll see an overview in a new page in which you can add absences manually. An overview of the national holidays are already listed by default.

To add your own; Scroll all the way down and select the "create new absence" button at the bottom, right corner of the page.