The phrase once in a blue moon pertains to an event that may happen very rarely or seldom or may not happen at all..After all when was the last time you saw the moon turned blue? The "blue moon" expression is old and dates back to medieval England. According to modern folklore, a blue moon is the second full moon in a calendar month. Usually, months have only one full moon, but occasionally, a second one sneaks in. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a singe month. This happens every two and a half years, on average.
For example, in 2004, the month of July had two ful moons. The first happened in July 2, while the other one happened in Juy 31, which by definition is a "blue moon".
