Project calendars provide a framework for scheduling projects and allocating resources. By default, projects are assigned the default Organization calendar and inherit that calendar's working days and hours and events. You can override, or customize your project calendar by adding events and modifying working days and hours (see the Actions > Edit Calendar menu option in a project's Settings section). Any events you add or changes you make to working hours will impact your project schedule. You can also switch the base calendar to any active calendar.
For example, you might decide not to schedule any work on Fridays for the first two months of a project to allow buffer in the project schedule while your resources get up to speed. You can set a recurring exception to the base calendar where every Friday for the desired months is a non-working day. When you schedule tasks, none will be scheduled on the Fridays set as non-working days.
Project calendars are viewable by anyone who can view the project, but can be edited only by users who can edit the project.
If changes are made to a project's base calendar, both project tasks and allocations may need to be rescheduled. For example, imagine a company decides to shut down for a week over Christmas and adds that event to the base Organization calendar. The project will need to be explicitly rescheduled to apply this change to the project schedule for both tasks and allocations (the event may or may not affect the schedule). See Synchronizing Allocations After Updating the Calendar.
If changes are made to a project calendar, task date changes are incorporated into the schedule automatically.
To reschedule tasks on a project
Determine which project(s) needs rescheduling. You can build a filter using the "Requires Rescheduling" filter field or run a report using the "Requires Rescheduling" report field, to determine which projects need rescheduling.
Navigate to the project's Task tab and select Actions > Reschedule.