The word Blue Moon identifies
two different kinds of moons.
The first term applies to the second in a calendar month with
two full moons, the latter of which occurred on March 31 , 2018.
In the space of a single season, the second term is used to define the
third of four full moons, with the next one expected to occur on August 22, 2021.
However, the blue moon that we’re going to have this Halloween applies to the
first meaning and, sadly, doesn’t mean that the moon is going to be actually blue.
The appearance of the Blue Moon takes place seven times every 19 years,
and until 2039 the next one will not be visible worldwide on Halloween.
During the years of World War Two,
the last time this occurred was in 1944.