A brief history of calendars

Q: “Once in a blue moon” is a rare event. But what does “blue moon” really mean?

Don’t fall for the trendy answer.

This was the final question last week at Pub Trivia, and our team won the evening with the same answer to this question that almost everyone else gave, probably the answer you’ve heard before: a blue moon means two full moons in one calendar month. This month, December 2009, has a blue moon on the 31, since it also had a full moon on the 2nd. But I had the nagging feeling that I’d read or heard somewhere (probably on QI) that the popular definition is wrong, that the real blue moon isn’t that straightforward. When I got home, Google confirmed it: We were wrong, quizmasters and all.