Someone here might have experience with this problem (it's not my personal problem and never has been):Say you have a customer who is an observant Jew and he wants his site to be offline somehow (or at least online but unavailable for surfing) during Saturday - Shabat in Hebrew, Sabbath in English.Is there a way to achieve this goal without causing SEO problem of any kind to the site?The way I can think of is a full-screen Lightbox that will cover all the site with a small message, but might not actually hurt the SEO as the site itself will be online but without surfing. 
Say a customer is an observant Jew and wants his site to be offline on Shabat - SEO problem?
seo;availability
Placing a website on a temporary hold should it be downtime for maintenance or for religious reasons will not harm your on-going SEO. It may however lower search results temporary until the website is backup, but obviously this should be expected, but long-term, no problem. The tip to placing a website on hold temporary without any serious impact on SEO is to use the correct status code. In all scenarios its always best to return a HTTP header response of 503 (Service Unavailable). Preferably it's best to use 503 without using a 302 redirect first, this is because the bots will see the status of 302 before it detects the 503 (good point by w3dk).This can be done either in the HTML, PHP or HTACCESS. Since the majority of answers currently cover various htaccess methods I thought I'd add a PHP method in case someone will prefer an alternative solution.Setting Header 503 Status in PHP$protocol = HTTP/1.0;if ( HTTP/1.1 == $_SERVER[SERVER_PROTOCOL] )  $protocol = HTTP/1.1;header( $protocol 503 Service Unavailable, true, 503 );header( Retry-After: 3600 );The Retry-After is in seconds, so 3600 is 1 hour. If you need more downtime then simply change the number to whatever you require. Simply Google how many seconds in X hours.Varnish and other caching setupsIf using a Varnish or any other setup that caches HTML, then you will need to purge the cache for the changes to take place as headers are generally cached.