Set off-days for one 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 working hours” 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.
Set off-days for all your projects
In your profile, you can also set off-days for all your project.
Click here to know more.