It's not just a song. The term "Blue Moon" is the term used when two full moons occur within a calendar month. The second full moon is called a "blue moon."
The month of August brings us not one, but two full moons. Ironically, the first will be on August 1st, followed by a second on Aug. 31.
HOW THE TERM CAME ABOUT
The phrase "once in a blue moon" was first noted in 1824 and refers to occurrences that are uncommon, though not truly rare. Yet, to have two full moons in the same month is not as uncommon as one might think. In fact, it occurs, on average, about every two and a half years.
In 1999, it occurred twice in a span of just three months. However, the next two moon month won't be until July 2015.