How Often Does a Blue Moon Occur?

A blue moon is the second full moon in a calendar month and occurs roughly once every 2.7 years, according to The term is used in the English language to refer to something that happens infrequently; it is said to occur "once in a blue moon."

A full moon during any month may appear to the naked eye to take on a slight blue tinge if there has recently been a forest fire or volcanic eruption near the viewing area. This effect is caused by smoke and ash in the atmosphere and does not represent the actual color of the moon.