Typically when you or I leave your home for a period of time, especially a vacation, you lock the doors, set the alarm and try not to let people or spiders into your home. But that rule doesn't have to apply to your web site.
John Mueller at Google posted a PSA, public service announcement, on Google+ saying you don't need to do that. He said:
PSA: If you go on vacation, you can leave the website up, and you don't need to noindex it.
Of course, you can imagine the SEO community having a field day with this PSA. Here are some of the responses:
Or you type at the top of your robots.txt::
User-Agent: Vacation
Disallow: /
The italian Parliament is recently discussing a law that allegedly imposes the shutdown of ecommerce websites on certain bank holidays
nofollow me into my holidays, thx :D
So feel free to leave the front door open, even when you are away.
Forum discussion at Google+.