Explaining Idioms: 4) once in a blue moon

The phrase 'once in a blue moon' means 'very rarely'. We use it to describe events that seldom occur.

This picture could be photoshopped, because the moon almost never looks blue.

 

Since the lunar calendar does not fit exactly into the standard calendar, it sometimes happens that there is more than one full moon in a single month. The second full moon is called the blue moon.

There is another, more traditional, meaning of the phrase 'blue moon' though. In old farming terms, a blue moon is the third full moon in a season. There is, then, one blue moon in each season.

However, to make matters more confusing, there are rare times when the moon does appear to be blue, such as in the picture above. This apparently happens because of 'unusual atmospheric conditions'.

The thing is, no one really thinks of these background meanings when using the phrase. When we say "that only happens once in a blue moon", we simply mean "that rarely happens".

Examples: 

• My football team is so bad. We only win once in a blue moon.
• Steve only sees his family once in a blue moon, because he almost never goes home.
• Q: "Do you come here often?" A: "Nah, only once in a blue moon."

In Popular Culture: 

In the Country song below, Edie Brickell sings about things happening "once in a while, once in a blue moon". Of course, we think of 'once in a while' meaning something happens more frequently than something that happens 'once in a blue moon'.