Bad Blue Moon Rising — A blue moon will rise tonight, May 21.
A blue moon is not the orb’s color but the term used to describe the third moon of a season — in this case Spring 2016 — in which four will appear.
Usually, seasons as they are a quarter of the year, have just three moons so the blue moon while not rare is not the norm. They happen about every three years.
Another definition describes them as being the second full moon of a calendar month, which is vague as it depends on time zones and what calendar one is using.
Bad Blue Moon Rising