Today is the Blue Moon—the second full moon in a calendar month.
Blue moons happen about seven times every nineteen years.
Posted by: MIKE JOHNSTON
Featured Comment by Doug (seconded by many other NPR listeners): "Based on a story on NPR last evening, it seems that this was not a Blue Moon and that people have been using the wrong definition since 1946 when it was incorrectly reported in Sky and Telescope magazine.
Idle Response by Mike who actually knows nothing about it: Doug, perhaps that will end up being one of those "errors" that are sanctified by popular acceptance into becoming true. For instance, there is (or was) no such word as "troops"—"troop" (or troupe) is already plural; the singular is "trooper." But I doubt you could convince many Americans, or even many lexicographers, of the non-existence and/or incorrectness of "troops" as a legitimate English word.
(I'm hoping the same thing isn't going to become true of "loose" for "lose," which I think is one of the most persistent misspellings on the internet. Or maybe it just annoys me the most.)
As for Blue Moon, we would probably need the AHED Usage Panel's scientific advisory panel to render a verdict on this one.
Further Comment by dasmb: "I've a degree in rhetoric and agree with Mike—the only definition of a term that matters in terms of effective speech is the one that your audience expects. Dictionaries are a largely academic thing—it doesn't matter if your usage is right by the dictionary, if it contradicts popular belief then it's unsuccessful speech.
"As for me, I'm going to celebrate this lunar event falsely called a Blue Moon with a nice tall glass of Blue Moon, a beer falsely called a Hefeweizen."