This is useful to warn users that the wiki will go in maintenance mode in some near future for example.

Recommended Solution

Use the "After Header" UI Extension Point.

• Create a page (any page) and add an XObject of type XWiki.UIExtensionClass in it.
• Fill the data as described in the "After Header" UI Extension Point page.

For example:

Old Solution

• Add a titlevars.vm field to the XWikiSkins class (type textarea, Text editor)
• Customize the current skin (from the "Administration>Presentation" page)
• Copy the content of http://localhost:8080/xwiki/templates/titlevars.vm to the titlevars.vm textarea field.
• At the end of this field, add for example:
  #error("This wiki will be down on 29th of October 2011, 10AM GMT+1.")