How can I set an “off” day in the calendar for a professional project?

You can manage “off” days in your professional project settings. Project>   “Settings” icon in the top right corner of the window. Go to the “Project calendar” section.

You can create “off” days in order to manage public holidays for instance, without changing your work schedule. To do this, click on the button “Add an ‘off day’” and select the chosen date in the calendar. You can select as many dates as you want. The “off” days will be automatically taken into account when you are scheduling tasks in your project.

Note: These settings are applied to the entire project, for all members assigned to the project.

For further information, you can take a look at this article on working hours and off days management.