When a non-working day is removed from a calendar and the calendar is saved, any schedule in the Scheduling module that is impacted by that change is automatically recomputed.
From the Setup menu, select Workday Calendars.
Or, click on the Workday Calendars icon on the Setup toolbar.
If necessary, select the calendar from which the non-working day needs to be removed from the Calendar drop-down menu.
Click on the holiday to delete.
Select Delete Entry from the Workday Calendars drop-down menu, or click on the 
Delete Entry icon, or right-click on the date and select Delete Entry.
A confirmation screen appears. Click Yes to delete the selected non-working day. Click No to cancel the deletion process.
Click on Save.
Click Close to exit the screen.
Defining the Company Work Week
Defining the Division Work Week