Schedule Windows Updates exclusively on a specific work day (and time)?

If you mean on an individual server, you can just set that up in Control Panel - Automatic Updates. It works the same way as on a Windows XP/Vista machine, where you set Automatic - Every Sunday at 1:00 AM

This works on my Windows Server 2003 machines

There is solution if you would use the WSUS. The WSUS works like a proxy between Microsoft Update Server and you.

The WSUS is able to collect all needed updates (with or without downloading them). Any client computer (or a server machine) receive updates only when they get approved by you. So you could approve all updates on saturdays for the next update time, which is scheduled at 1 a.m.