Google says - "Outages that are not clearly marked as such can negatively affect a site’s reputation."
In that scenario due to the ongoing site maintenance, or legal or cultural requirements, there are methods to deal with the planned website downtime in a way that will generally not negatively affect your site’s visibility in the search results.
How to avoid the negative impact on website's visibility during the 'Planned Site Downtime' in Google Search Results?
Instead of returning a 404 Not Found HTTP result code or showing an error page with the status code 200 (OK) when a page is requested, it’s better to return a 503 HTTP result code (Service Unavailable) which tells search engine crawlers that the downtime is temporary.
Moreover, it allows webmasters to provide visitors and bots with an estimated time when the site will be up and running again. If known, the length of the downtime in seconds or the estimated date and time when the downtime will be complete can be specified in an optional Retry-After header (http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.37), which Googlebot may use to determine when to recrawl the URL.
Returning a 503 HTTP result code can be a great solution for a number of other situations. We encounter a lot of problems with sites that return 200 (OK) result codes for server errors, downtime, bandwidth-overruns or for temporary placeholder pages (“Under Construction”). The 503 HTTP result code is the webmaster’s solution of choice for all these situations. As for planned server downtime like hardware maintenance, it’s a good idea to have a separate server available to actually return the 503 HTTP result code. It is important, however, to not treat 503 as a permanent solution: lasting 503s can eventually be seen as a sign that the server is now permanently unavailable and can result in us removing URLs from Google’s index.
header('HTTP/1.1 503 Service Temporarily Unavailable');header('Retry-After: Sat, 8 Oct 2011 18:27:00 GMT');
If you set up a 503 (Service Unavailable) response, the header information might look like this when using PHP.
Suggested Reading: Status Code Definitions