The phrase “once in a blue moon” is meant to describe something that’s very rare, although in reality blue moons happen once every three years or so. And actually, there’s nothing blue about a blue moon — rather, it’s a term used to describe the second time a full moon appears in a given month. The last time we had a blue moon was in 2012 and we’re not scheduled to get another one until 2018.
Read more here:: Boy Genius Report