Once In a Blue Moon

NCE

Example: I visit my family in India once in a blue moon.

Origins of "Once In a Blue Moon" phrase

The 'blue moon' expression with the 'impossibility' meaning is old and dates back to medieval England while the current meaning dates back to the early 1800s in The Maine Farmers' Almanac.

A “blue moon” is a real astronomical phenomenon, referring to the second full moon in the same month.

It's a rare occurrence as it happens about every two or three years.

Usually, a blue moon just looks gray or white like any other full moon, but on even rarer occasions, the moon actually does seem to be blue.

According to NASA, volcanic eruptions or forest fires, the oils in smoke can make the moon appear blue. 

The 1883 Krakatoa eruption portrayed in the background picture produced blue moons all across the globe for nearly two years.