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.
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? :-)