It is important to add non-working days to your schedule in order to accurately evaluate the actual progress of your project versus your schedule. On the Settings page for your project, your will be able to customize what these days are and when they occur.
In order to add a holiday, scroll down to the Project Calendar section and you will see the 'ADD DAY +' button.
Once you click on it, you will be asked to provide the name of the holiday and its date. Please be aware this process needs to be performed individually for each one of the holidays that occur throughout the length of your project's duration.