More or less the same thing that Project Banner
All mockups : https://www.figma.com/file/q9hUV9GHuCVmIOCZRzcPqp/MOTD?node-id=0%3A1
As for project banner:
The banner is limited to 1 line (no multiline) and limited formatting (bold, italic, links). It's pure html with only bold, italic and links (ckeditor for project admin).
The banner history is not kept
The banner can be changed with REST API
End users can collapse messages
The site wide banner has an "importance" that can be used to weight messages