Unscheduled?

I got the following email in my NUS mailbox:

Dear Students,

There will be an unscheduled system maintenance on (Sat) 21 March 2009, 8pm to (Sun) 22 March 2009, 1am.

During this period, Email Archival Service will not be available. If you try to retrieve archived emails from your mailboxes, you may receive an error message box as follows:

.. .. blah blah blah .. ..

This maintenance is necessary for NUSmail service stability.

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I don't see how this is unscheduled.