I believe that exactly what you want will be available in 2.0 release with fully implemented playlists.
In current release (1.8.10), you’d need to either manage the “standard” layout ie add / remove the announcement images on the required dates manually (or via API), or have the announcement images in a separate layout scheduled to display with a specific to date with the same priority as the standard layout, so the player cycle through all of them.
In 1.8 series, there were additions to the Schedule which could help you - https://xibo.org.uk/manual/en/scheduling.html
However as I mentioned, exactly what you want will come in 2.0 series with playlists.