The Secret Puzzle Hunt Cabal is a treasure. You can tell 'em "I'm thinking of running a hunt on the 13th" and, between them, they know when the MS Intern Hunt is, the NPL convention, the MS Puzzle Safari, … And thus you may find yourself running a hunt on the 21st, instead. A date that looks about the same on the calendar, but doesn't exclude umpty-ump percent of the likely participants is a good thing.