With the menu option Edit - Holidays you can determine days, which will be indicated as holidays in the Attendance Planner.
Holidays are usually locked for input. If you would like to make entries for these days uncheck the checkbox Lock non-business days and holidays for input.
You can extend the list with the button More Holidays and enter further annually recurring holidays.
You can also mark days as once-off holidays. To do this go to the main form and right-click the mouse, whilst pressing the CTRL key, in the headline of the desired day.
You can transform the indicated holiday back into a working day, by right-clicking the mouse, whilst pressing the CTRL key, in the headline on the appropriate day again.
If you have already made entries for days, which you have subsequently changed to holidays, you have to correct these manually.