How to serve a custom error page if the server is down?

I have a website and want to serve a custom error page to my users if my server goes down. If the site is unreachable, like to say something like "we will be back soon for you". Is this possible in general or am I not able to serve anything without my server is up? If its possible how do I configure this?

I have a server hostet with Ubuntu 18.04 as OS and nginx as webserver.

Thanks you :)