Blue moon

This definition is from the website page -  link below.
Most definitions say that a blue moon is the second full moon in a calendar month. This year, July’s first full moon was on the 2nd. Because there are 29.5 days between full moons (known as the lunar month), here we are again at full moon on 31st.
http://www.theguardian.com/science/across-the-universe/2015/jul/31/blue-moon-how-to-see-tonights-rare-event