Archived - Mac OS X's Shut Down schedule will only work when one user is logged in

With Mac OS X 10.3 or later, if no user is logged in, or if more than one user is logged in at the same time, the computer cannot shut down automatically at a time specified in Energy Saver Schedule preferences.

If you want to schedule your computer to Shut Down automatically at a scheduled time, be sure that one—and only one—user is logged in.

Mac OS X 10.4 tip: Update to Mac OS X 10.4.6 or later to allow the computer to shut down at the scheduled time while at the login window.

Check out Mac OS X 10.3 and later: Computer must be awake for scheduled shutdown to occur, too.