It is inevitable that once you launch your web site there will be future structural, aesthetic or database changes requiring the site to be taken offline, site maintenance notices communicate to your site visitors why your site is not available during these times.
A standard template I use to display web maintenance notices to site visitors is included below. Tweak it to your liking and recommend improvements in the comments.
[Site name] is undergoing site maintenance
This site is currently down as it undergoing site maintenance. Don’t worry though, as [Site name] will be available soon! Please check back again shortly.
If your enquiry is urgent or problems persist, please contact the system administrator of this site.
WordPress site owners can install the plugin Maintenance Mode to enable and disable a custom web site maintenance notice during downtime. If you’re a site owner using another web platform what are you using for maintenance notices?