Coming soon, or maintenance

These are not two names for one thing, and picking the wrong one is expensive.

Coming soon is a site that does not exist yet. The page answers 200, because the address is meant to be found and shared while you build.

Maintenance is a site that exists and is briefly away. The page answers 503 Service Unavailable with a Retry-After header, which tells a crawler to come back rather than to treat your holding page as the site's new content. A fortnight of 200s on a maintenance page is how a site loses rankings it spent years earning.

The Retry-After is worked out from your opening time when you have set one, so the number is true rather than a guess. Without one it says an hour; further away than a day it says a day, so crawlers keep checking daily rather than staying away until a date weeks off. Google's advice is to answer 503 for a day or two at most; for a shop closed longer than that, Shop closed for orders below keeps the pages up and stops the sales.