It'll be also great if such a message had a link to blog post (if any) describing upcoming maintenance.
Currently such messages looks like:
We'll be doing some network maintenance today starting around 2PM UTC. The site may go up and down while we shuffle things around.
In just a bit, Stack Overflow will be offline for 30 minutes while we give our database hardware some love.
Both of those have strong issue with time specified for maintenance since there is no date in the message, in case such message wasn't removed it confuse people a lot.