In the Configuration tab you indicate the working days of your company. The checked days are by available for planning in the scheduling board. For certain actions, such as splitting cards, only available days are taken into account.
You can set a specific date on unavailable dates in the planning board, for example, holidays. Read here how to make a specific day unavailable in the scheduling board.
Through the filter function, you can decide whether you want to show or hide unavailable days in the planning board.