The saying, "once in a blue moon," originated back in 1528, in a pamphlet attacking the clergy. Translated from Old English it reads, "If they say the moon is blue, we must believe that it is true." It meant simply something that was impossible, something that would never happen. Not long after another phrase became popular, "They would make men believe that the moon is made of green cheese."
Now the phrase means something that happens only once in a while, a rare occurrence.
Which is what we will see tonight, something that happens, but not all that often. It is the kind of blue moon most of us are familiar with; a second full moon in a month.
There are several different kinds of blue moons. As I started to research and read about them, it got a bit confusing. Suffice it to say, the blue moon we will see tonight is not the only kind of blue moon that appears in the sky. You can learn more about them here.
Have a safe, happy and Blue Moon New Year!