A "Blue Moon" is a fairly infrequent phenomenon involving the appearance of an additional full moon within a given period. But which period ? there are two definitions of the term, and one was borne out of a misunderstanding of the other.
The older meaning defines a Blue Moon as the third full moon in a season that has four full moons. Called a seasonal Blue Moon, this occurs about every 2.5 years, according to NASA. Why the third moon? There are roughly 29.5 days between full moons, making it unusual for two full moons to fit into a 30- or 31-day-long month. (This means that February will never have a blue moon.) Seasons normally have three full moons, and some of them, for traditional and religious reasons, must occur at specific times of the year. So, the "Moon Before Yule" is always the one before Christmas.
The other meaning is that a Blue Moon is the second full moon within a single calendar month. This definition ? a "monthly Blue Moon" ? has gained popularity in recent years because of a misinterpretation of an almanac's original definition.
Which one is correct? Well, since language and the meaning of words constantly evolve ? take your pick. Both are commonly used today and either definition is considered valid. As Texas astronomer Donald W. Olson wrote in a 2006 column for Sky & Telescope magazine, "With two decades of popular usage behind it, the second-full-moon-in-a-month (mis)interpretation is like a genie that can't be forced back into its bottle. But that's not necessarily a bad thing."
Once in a Blue Moon