This question is an exact duplicate of:
I've noticed that a new "Community Bulletin" box is being rolled out network-wide:
Can someone describe this new feature?
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The Community Bulletin box is a tool for the community to promote events and initiatives that are important to them. On-site activities like tag cleanup events, chat events, and weekly topic contests are some examples of things to promote in the community bulletin.
Moderators and select Stack Exchange employees can add new events from the Mod Links menu, giving each entry a title and beginning and ending date & time. Each entry must be linked to a Stack Exchange property (a Stack Exchange main site or meta post, Blog Overflow blog post, or a post on the main Stack Exchange blog).
If no entries have been manually submitted to be displayed, the Community Bulletin will be populated with featured and active meta posts.
It is currently live on Stack Overflow, Super User, Server Fault, and a select number of SE 2.0 sites, with network-wide rollout occurring in the near future.
The new Community Bulletin box is an enhanced replacement for the "Visit Meta" box previously used. It is now displayed near the top of the side area by default and can include links from Meta Stack Overflow as well as from the site's own meta (if not Stack Overflow).
The box also emphasizes more important, network-wide meta topics. Moderators can add community events to the community bulletin box from Mod Links menu.