Originally, “once in a blue moon” referred to an absurdity, according to the Oxford English Dictionary. The OED notes a reference to a proverb from 1528 that says “If they say the moon is blue, we must believe that it is true.” And while the moon is not actually blue, it can appear blue.
It happens only rarely, when small particles in the atmosphere scatter more red light than blue, allowing less red light to be seen from the ground. This makes the moon temporarily appear blue. Although this phenomenon is quite rare, it has altered the meaning of the phrase “once in a blue moon,” which is now commonly understood to mean “every now and then.”
The term also often refers to the second full moon in a given month. In an article published in the Sky & Telescope magazine in 1946, James H. Pruett used the phrase to refer to the second full moon to occur during a given month. While this was a misinterpretation of the original meaning for “once in a blue moon”, after a couple of decades of popular use (or should that be misuse?), we now also consider the 13th full moon in a year to be a “blue moon.”