Project Calendars allow Resource Usage reports to take note of exception days. Generally, most Project Management templates will use the default Project Calendar; however, you may have a need to add an additional project calendar. To do this first you will need to create a standard SharePoint Calendar and then enable it as a Project Calendar.
There are two types of exception days:
(1) Working Time
(2) Non-Working Time
To create a Calendar:
Click Settings | Add an App.
Enter a Name.
To enable a Calendar as a Project Calendar: