http://www.newsweek.com/blue-moon-may-full-flower-moon-1424728

On May 18, a full moon will appear in the night sky. This year, May’s full moon, known as the "full flower moon," will also be—according to one definition—a “blue moon”—a celestial event that happens once every two to three years.

But what is a blue moon? And will May’s full moon be one? There are two definitions of what a blue moon is. Under one, the full flower moon is not a blue moon. Under the other it is.