Please use the following steps to add a holiday to a calendar:

1. Go the "Calendars" tab in the editor by clicking "Calendars" on the sidebar on the left-side of the screen.

2. Double-click on the calendar to open the "Calendar Properties" for the calendar that you wish to edit. This would most likely be "Standard" calendar if you are using the defaults.

3. Click on the date in the calendar and then click "Clear" next to "Working time". The calendar will now show this date as red.

4. Click the "Save" button on the Calendar Properties