If you want to add the day of the week, to the date/time on the bottom right of your display in the notification area, add ddd to the short date field as shown in the below screen shots.
You can add dddd but if you find the day of the weeks is not showing because of insufficient room in the notification area, then change the dddd to ddd
Start by typing Control Panel in the search field in the bottom left of your display. Click on the top result in the search results.
