It’s the second full Moon of the month that is often referred to as “blue Moon”.
That doesn’t mean the Moon will actually be blue in color. The term is referring to the cycle of two full Moons in one month. Typically, there is only one each month. It’s the irregularity of two full Moons in one month that inspires the reference, “blue Moon”.
Ever hear the phrase “once in a blue Moon”? You might be surprised to learn the phrase has a long history but it’s only within the last 25 years that its meaning has expanded to include occurrences of two full Moons in a single month.
“According to old folklore,” some people say, the second full Moon in a calendar month is called a “blue Moon.” They go on to explain that this is the origin of the expression “once in a blue Moon.” But it isn’t true! The term “blue Moon” has been around a long time, well over 400 years, but its calendrical meaning has become widespread only in the last 25 years.