The phrase 'once in a blue moon' is frequently used to refer to an event that occurs very rarely, but what exactly is a blue moon? In the simplest of terms, a blue moon is the second full moon in any calendar month. Note that despite the name, the color of the moon does not change during a blue moon.
The average moon cycle lasts about 29.5 days, and of course most months have 30 or 31 days. Its clear then, that blue moons are relatively rare. For a blue moon to occur, a first full moon has to fall at the very beginning of a month so that the next full moon (29.5 days later) can rise during the same month.
On average, there are about 2 and a half years between blue moons. Of course the months that have 31 days are much more likely to have blue moons than those that have 30 days. February, the only month with fewer than 30 days, never has any blue moons.
About four times per century, a calendar year has two blue moons. 1999 was the most recent year with two blue moons. The 21st century will have the following double-blue-moon years: 2018, 2037, 2067 and 2094.
There is a little controversy over what a blue moon actually is. We have described the modern version, but in previous centuries, the term blue moon referred to third full moon in a season that has four full moons.