Blue Moon
The next blue moon is on October 31st
A blue moon is an additional full moon that appears
in a subdivision of a year: either the third of fourth full moon
in a season, or a second full moon in a month of the common
calendar.
The phrase in modern usage has nothing to do with the actual
color of the Moon, although a visually blue Moon (the Moon
appearing with a bluish tinge) may occur under certain
atmospheric conditions for instance, if volcanic eruptions or
fires release particles in the atmosphere of just the right
size to preferentially scatter red light.
Read more on Wikipedia