Can I customize the OSX screen saver to show the Day of the Week?
This can't be done with any of Apple's screen savers that ship with OS X. 

My favorite screen saver is the Marine Aquarium ($20) screen saver which has a customizable crystal that can display the time (analog or digital), date, or a calendar.

If you Google "os x screen saver with date" you'll come up with many free or inexpensive alternatives. My favorite among these is the Pong Saver, but that doesn't provide the date.