What is a Blue Moon?

We all know that "Blue Moon" has nothing to do with the color of the Moon.  A "Blue Moon" is some infrequent phenomenon which involves the appearance of an additional full moon within a given period.  The period is something like two full moons appearing within a month's time.  

Normally there is a gap of 29.5 days between two full moons so it is very unusual for two ful moons to fit into a 3o or 31 day-long month ( means February will never have  a blue moon).  So a blue moon is a second full moon appearing within a single calendar month. This is the meaning of a blue moon.  So a blue moon does not have associate blue color but a blue moon is a full moon which appears within the same calendar.

The next monthly Blue Moon will be on 31st March, 2018.  The previous one was the Super Blue Moon on 31st January, 2018.  

Origin of the term "Blue Moon".

The phrase "Once in a blue moon" has been around for more than 450 years according to Philip Hiscock, a folklorist at the Memorial University of NewFoundland.  In a 2012 article in Sky&Telescope magazine, he explained that the earliest use of the term was much like saying the moon is made of green cheese - It indicated some thing absurd.  "He would argue that the moon is blue" was similar to saying "He would argue that black is white".