I learned today that the term, "Blue Moon" does not, in fact, mean a full moon twice in one month, but rather the third moon in a season with four full moons. To quote Wikipedia (and to summarize our wonderfully thorough (insane?) weatherman on WORT):
"Blue moon is also a name given to the third full moon of four during a given season. The Farmer's Almanac also defines blue moon as the third full moon of four that occurs in a season... From 1946, people started calling a full moon a blue moon if it was the second of two full moons to occur in the same calendar month. This definition of blue moon originated from a mistake in an article in the March 1946 Sky & Telescope magazine, which misinterpreted the dates & meaning provided by the Farmer's Almanac."
Which means that right now, we have a blue moon. So, Dad, you said you would come to visit, "once in a blue moon." I'm waiting.