Once in a blue moon

Blue moon over Brooklyn. This is the third full moon in a season with four full moons.
There are two kinds of blue moons, & each kind by itself is quite rare, which is why we
say ONCE IN A BLUE MOON when we talk about something that very seldom happens. 

I was PSYCHED (excited) to learn just a few hours ago on my way home that tonight's full moon is actually a blue moon! I teach the expression ONCE IN A BLUE MOON in the classroom quite a bit, so I was so excited to finally see one and take a photo of it. If you're not familiar with the expression ONCE IN A BLUE MOON, it's an adverbial phrase that means "very rarely." For example, I take a taxi ONCE IN A BLUE MOON. Also, living in New York City, I drive just ONCE IN A BLUE MOON.

I quickly did some research on the train when I found out about tonight's blue moon and learned that it has two definitions. The more common definition and the one I tell students describes a blue moon as the second full moon in a calendar month. This was apparently a mistake made by a magazine in 1946 that simply STUCK (stayed; remained), according to the Huffington Post. The original definition of a blue moon states that a blue moon is the third full moon in a season that has four full moons. A season usually only has three full moons, so one with four is quite rare. 

A close-up

Tonight's blue moon is based on the original definition. The last one of this kind happened on November 21, 2010, and the next one won't be until 2015.  It's a rare phenomenon, which explains why we say ONCE IN A BLUE MOON. So, what do you do just ONCE IN BLUE MOON?  :-)