I have a suggestion which would solve the problem in the short term (though it could become a big problem itself if too many events were occurring on a daily basis):
Use the petition system to ask for an announcement to be made giving date, time and place; the announcement can then be made on the appropriate day in the MOTD. That way everyone knows something is going to happen when they log in.
eg.
MOTD
Anonymous is holding a party today to celebrate his birthday.
Meet in Brado's Tavern, Ojaveda at 5.30pm gmt
Anyone wishing to take part in an event being run by Anonymous Guild
Should gather in the Plaza at 6.30pm gmt today
I would have thought that this is exactly how Message of the Day should be used instead of seeing the same thing for weeks on end, and by going through the petition system you avoid abuse.