503 503 Service Unavailable

The server is temporarily unable to handle requests, often because it is overloaded or under maintenance.

When you'll see it

When the server is temporarily unable to answer: planned maintenance, overload, or an application still starting up.

Send a Retry-After header during maintenance, so clients and crawlers know when to return.
Use 503, not 200 or a redirect, for a maintenance page. It tells search engines the state is temporary.
Resource limits and traffic spikes if it wasn't planned.
Don't leave it on for long. Search engines may start dropping pages after a few days.

HTTP/1.1 503 Service Unavailable
Retry-After: 1800
Content-Type: text/html

Google retries later. Short 503 outages are fine. Extended 503s lead to deindexing. Use a Retry-After header when you can.