The Straight Dope covered this awhile back: The "two full moons in a month" definition is recent and somewhat bogus, like the bit about the seasons officially

The Straight Dope covered this awhile back: The "two full moons in a month" definition is recent and somewhat bogus, like the bit about the seasons officially starting on the solstices and equinoxes. Originally "once in a blue moon" meant "pretty much never", and the modern definition stems from an error made writing a question/answer for Trivial Pursuit. Full article: http://www.straightdope.com/columns/read/162/is-there-ever-really-a-blue-moon http://www.straightdope.com/columns/read/162/is-there-ever-r...