The saying "once in a blue moon" comes from the fact that a blue moon rarely happens.  The common definition of a "blue moon" is two full moons that occur in a single calendar month.

When Meg told me that tonight's full moon would be a "blue moon," I didn't believe her.  I wondered, "how can you have two full moons in the same month when one happens on the 20th?" 

Well, I found that there is also a seasonal definition of a blue moon...the third of four full moons within a season...which tonight's will be!   ~Murphy

CLICK HERE to read more about blue moons.

Photo by Beth Swanson | Shutterstock