Calendar is very important in project because scheduling is done based on it. In Primavera P6 you can create and assign calendars to each resource each project and each activity. The three types of calendars in primavera p6 are called Global, Project and Resource.
You can create and assign calendars to each resource and each activity. These calendars define the available workhours in each calendar day. You can also specify national holidays, your organization’s holidays, project-specific work/nonworkdays, and resource vacation days. Calendar assignments are used for activity scheduling, tracking, and resource leveling. Whether an activity uses its assigned calendar or the calendar of an assigned resource depends on the activity type you specify.