HR users can define new days that will not count to used days when creating new Vacation Document. Every new created holiday can be defined as one time only, or recurring. In case of workdays - days of week.
You can access this panel as HR User from Navigation Bar Menu ➜ Management (1) ➜ Holiday Schemes (2). Now you can edit a Default Holiday Scheme, or create a new one to edit it, the same way as the default one (4).
By editing selected holiday scheme you should see a similar view, like the one below: