The term has traditionally referred to an “extra” full moon, where a year which normally has 12 full moons has 13 instead. The “blue moon” reference is applied to the third full moon in a season with four full moons, thus correcting the timing of the last month of a season that would have otherwise been expected too early.
This happens every two to three years.
Another meaning of Blue Moon is color due to some atmospheric phenomenon that gives the moon a bluish tint. This occurrence is extremely rare.
Owing to the rarity of a blue moon, the term “blue moon” is used colloquially to mean a rare event, as in the phrase “once in a blue moon”.
The next Blue Moon will occur on January 31, 2018.