I aim to please. I know you know about it. Maybe many don't.

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 2.7 years. Why the third 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.