In WSN 9.0 or later, at Admin Panel -> Settings -> Switches you'll find an "announcements system" option. When that's enabled, you can set up announcements at Admin Panel -> Miscellaneous -> Announcements.

Announcements are simple messages displayed to everyone near the top of the page. Typically they're used to announce an upcoming event, planned downtime, or some sort of special offer. Simply type a title and message for the announcement and click to add it. You'll see your currently running announcements listed at the top of the Admin -> Miscellaneous -> Announcements page with the option to remove them when they're no longer relevant.

Each announcement will display to each visitor until the visitor clicks to dismiss it. For registered members, the database records which announcements they've dismissed. For unregistered guests, that information is saved in a cookie so they may see the announcement again when they visit from a different web browser / device.

You will not want to have many announcements running at the same time, as this would get annoying to people. Usually you'll only run one at a time.

To customize the look and feel of the announcement, go to Admin Panel -> Themes -> Manage Templates -> Announcements. If you want to change the location where that HTML gets inserted, go to Admin Panel -> Themes -> Manage Templates -> Wrapper and find [INSERTFILE=announcements]... cut and paste that to your prefered location. If you only want announcements shown on a specific page (like the front page or the member home main page), you can remove the [INSERTFILE=announcements] from the wrapper and put it in your desired other template instead.