'Once in a Blue Moon' this Friday

If you look up in the sky on Friday, you will see something that only occurs "once in a blue moon."

On Friday, much of the world will have the opportunity to observe a Blue Moon: The phenomenon that appears only once every three years, according to the National Oceanic and Atmospheric Administration. This means that you will not be able to see this again until January 2018.

Just because it is called a blue moon does not necessarily mean the moon will appear blue.
The moon only appears blue when smoke or dust particles are in the atmosphere, usually from a volcanic eruption or large wildfire.

By its modern definition, a blue moon is the second full moon in a calendar month, an astronomical phenomenon that occurs about once every 2.7 years.

The first recorded appearance of the phrase “blue moon” was in the 16th century. At this time, the idea of a blue moon represented absurdity; an inconceivable event. By the 1700s, the phrase had evolved to mean “never.” 
The phrase appears again in an 1821 with the meaning „a long time„ (I haven't seen you this blue moon.) and it was commonly used street slang. 

In a 1946 article in Sky and Telescope Magazine titled “Once in a Blue Moon”, journalist James Hugh Pruett incorrectly stated that a blue moon was the name given to the second full Moon that appeared in any given month. 

The phrase has become popular with businesses and products in recent years, with countless restaurants and shops incorporating “Blue Moon” into their name.