Definition
once in a blue moon
phrase

very rarely.

“he comes round once in a blue moon”

People have been using this phrase to imply “hardly ever” for at least 400 years.

 Original Meaning

It seems to have its origins in a rare meteorological phenomenon in which the moon’s light passes through dust or smoke particles, creating a bluish tinge similar to that caused by cigarette smoke.

 A New Meaning

However within the in the last 150 years or so the phrase has somehow acquired a second meaning :

A month in which two full moons appear- a calendrical quirk made possible by the fact that the average length of a month is just a smidgen longer than the period between successive full moons.

Quite why this new meaning has anything to do with the colour blue isn’t clear but it does at least allow us to give a precise definition of the phrase :

a blue moon puts in an appearance once in a period of slightly over 2 years.