When I started in Atlantia, in 1993, the rule was that no two events could be on the calendar on the same weekend if they were under 150 miles apart, unless the autocrat of the first event listed gave permission. Oh, and Kingdom events had first claim on the first weekend of every month. Oh, and you couldn't schedule an event more than a year in advance, I think it was. The result, naturally, was a scramble to get on the calendar as soon as it opened up for the month you wanted.

I don't know if this is still the case; they might have come up with a less stressful approach.