The basic idea is a sticky thread which orders its messages not by date of posting but by the date of the event.
I imagine a post would need an inline calendar, or date boxes to fill in to complete the post (required).
Most recent at top (could be future event).
On opening the thread the viewer is automatically shown today's dated events and pagination to look further ahead or go back.
This would only carry simple announcements as it wouldn't help looking things up to have the posts cluttered with discussion.
Perhaps there could be a link to open a new thread with the same title + the word "... discussion."
If this thread has been created the link then opens it as a normal thread.
Email notification on this thread goes to the original poster of the linked message on the Diary thread.