English Wiki's pop-up page?
Fragment of a discussion from Project:Support desk
You don't need Extension:DismissableSiteNotice.
Wikipedia used Extension:CentralNotice to distribute it from the central wiki, however you don't need that extension either.
The notice system itself is in MediaWiki core. Edit MediaWiki:Sitenotice on your wiki and put a the HTML for the overlay in there with CSS classes on them, and from MediaWiki:Common.css style them to be fixed positioned over the entire page and use display: none; to hide everything else.
