The result of a hackathon is google wave robot, called eventdator(I'm lousy at making up names):
What it does now is, it associate a event name with a date. Which stores in the appengine datastore. To use it:
1) Add the robot on the wave, address is firstname.lastname@example.org
2) The command to use is:
get:event namethen done
3) To set an event
add:event name,eventdate,trueset true if it is a holiday. else false.
I still open to feature request. And I still figure out how to have a better interface, etc.