How to display site wide notification to all users, even if they are not login.
I want to notify that site will go under maintenance on particular time.
You could also try adding the relevant notification text as raw HTML to the additionalhtmltopofbody config in the Additional HTML settings page at /admin/settings.php?section=additionalhtml.
This will then show for both people logged in and people not logged in.
In my example screenshot, the HTML relevant notification text also includes "<br><br>" spacing as the Boost theme would otherwise be hiding the text under its top navigation bar, so theme display settings have an impact on this feature too.
...which is what the third link suggested 😉
Has nobody mentioned the Announcements forum on the front page, the so called "Site news" - Site administration > Front page settings > (frontpage) and (frontpagelogin)?