Select the cells that contain dates that you want to show as the days of the week.
On the Home tab, click the dropdown in the Number Format list box, click More Number Formats, and then click the Number tab.
Under Category, click Custom, and in the Type box, type dddd for the full name of the day of the week (Monday, Tuesday, and so on), or ddd for the abbreviated name of the day of the week (Mon, Tue, Wed, and so on).
