When you create a project plan, Microsoft Project automatically attaches the default project calendar "Standard" to your plan. The Standard calendar shows all weekdays as workdays and all weekends as nonworking days. The default working hours for each workday are 8:00 a.m. to 5:00 p.m., with a break from 12:00 p.m. to 1:00 p.m. If you want a different set of working and nonworking days and hours, you need to modify your project calendar. For example, you can include national holidays.
To ensure peace among the people whom you schedule, you probably want to modify the Standard calendar so that it at least designates national holidays as nonworking days. Also, if your resources don’t work specific shifts, just make sure that the workday start and finish hours equal the total number of hours they work each day. For instance, if a resource works 9 hours per day, you might specify start and finish times of 8:00 A.M. and 6:00 P.M., with a 1-hour break.
Changes you make to a base calendar are reflected in the resource calendars that depend on it. Thus, if a national holiday will be taken by all your resources on a project, you should modify all the base calendars.