When my mom wanted to point out the rare or the unlikely she would use the expression “once in a blue moon” .  The music group Sha-Na-Na had a hit with the song Blue Moon and as rare as it might be the next blue moon is coming – tomorrow! Friday, August 31st is the night and if the sky is clear, you’ll be able to see the blue moon. However moons that appear blue are the results of dust particles in the atmosphere so don’t be disappointed if it looks like any other full moon. The phrase “blue moon” refers to rarity, not to color. It really means it’s the 2nd full moon of the month, the first one this month was on August 1st.

So what’s the big deal? It’s a somewhat rare phenomenon. The last blue moon was on New Year’s Eve, in December of 2009 – and the next blue moon won’t be until July of 2015.  So while August ends every year, it ends on a blue moon only once in a blue moon.

Source: http://www.livescience.com/22743-last-blue-moon-friday-2015.html