How can I broadcast a message to my wiki users, Eg. for maintenance reasons?

There is a special BROADCASTMESSAGE preference setting documented very briefly in DefaultPreferences.

You set this like any other site-wide preference in Main.SitePreferences, although BROADCASTMESSAGE can be set per-web (in WebPreferences) or even per topic (read about PreferenceSettings for more about scope of preference variables).

It adds a "broadcast message" to the top of the screen (depending on the skin used), for example, let's set a message that the site is going down for maintenance in a few hours:

   * Set BROADCASTMESSAGE = %WIKITOOLNAME% is going down for maintenance at %SERVERTIME{"$year/$mo/$day"}% - 16:00 (in %CALC{$EVAL($TIMEDIFF($TIME(), $TIME(%SERVERTIME{"$year/$mo/$day"}% - 16:00), hour))}% hours)