“Once in a blue moon” is used to describe something that doesn’t happen often. but what exactly is a blue moon? It’s a full moon that occurs in the same month as another.

Although it occurs every two or three years (on average about every 2.7154 years), this is the first time in almost 20 years that we’ve had one on New Year’s Eve.

Want more? Here’s some interesting information about this phenomenon:

http://news.nationalgeographic.com/news/2009/12/091230-blue-moon-new-years-eve.html

http://en.wikipedia.org/wiki/Blue_moon