“Once in a blue moon” is a common expression that has been used for a long time, and which means ‘not very often,’ or ‘very rarely.’ It often refers to an extra full moon; however, it has been used to describe the way the moon actually looked, when for different reasons it had turned a blueish color.

In America ‘blue moons’ were calculated every year and listed in the Maine Farmers’ Almanac starting in the 19th century and continuing on into the early 20th century. Instead of using the calendar year that we are used to, which runs from January 1st through December 31st, the almanac used what is called the ‘tropical year,’ which extends from one winter solstice, usually around December 20 (the “Yule”), to the next. Most tropical years contain 12 full Moons -- three each in winter, spring, summer, and fall -- and each of those full moons is named for an activity appropriate to the time of year (Easter Moon, Harvest Moon, Yule Moon, etc.). But every once in a while a tropical year contains 13 full Moons, so one season has four full moons instead of just three, hence the ‘blue moon.’ The almanac also names all the full moons, including blue moons, at the beginning of each tropical year calendar. But the interesting twist is that the blue moon is the third full moon in the season that has four. Why the third moon rather than the fourth, you might ask? That is because other full moons with other special names have to fall at their correct times of the year. This rule made it all work out on the calendar according to the almanac.

There are two astronomical definitions of a Blue Moon; both are a type of Full Moon. When the Moon very rarely actually looks blue, it's because of a certain size dust particles in the atmosphere.

The saying 'once in a blue moon' has nothing to do with color. Instead it refers to the relatively rare occurrence of what is called a 'blue moon'. (However, an actual blue-colored moon could occur from atmospheric effects such as a big forest fire on Earth causing a lot of haze. This has happened in the past!) The definition of a Blue Moon has changed over the years (see below). However, the commonly accepted definition of a Blue Moon is simple: it is a full moon that rises twice in one month. It happens because sometimes the number of days in a calendar month are greater than the cycle of the moon, which is 29.5 days. This can cause the moon to rise twice in one month, near the first and the last days of the same month. It is said that blue moons happen every 33 months or about every 3 years. This relatively rare occurrence has spawned the saying 'once in a blue moon'.

Once in a blue moon means very rarely. But just how rare depends on your definition.

In astronomy, Blue Moon is defined as either the third full moon of an astronomical season with four full moons or the second full moon in a calendar month. Such a blue Moon (second full Moon in single calendar month) will next occur on Friday, July 31, 2015 at 10:43 am UTC.

Contrary to popular belief, a blue moon is not actually blue in color. Blue moon is a term that is used to describe the third full moon of a season that has four full moons.

A year has four seasons - Spring, Summer, Fall (Autumn), and Winter - with three months and three full moons each. When one of the seasons in a year has four full moons, instead of the usual three, the third full moon is called a blue moon.

These days, the second full moon in a calendar month is also often referred to as a blue moon. This particular use was popularized due to a misinterpretation in a 1946 article in Sky and Telescope magazine. Such blue moons occur rather frequently - at least once every two or three years. The next such blue moon will occur on July 31, 2015.