What is blue moon?
The number of days between two full Moons is 29.5. This adds up to a little less then a full month. As a result, it is possible for a single month to have two full Moons. If this occurs, then the second full Moon in the month is referred to as a Blue Moon. A Blue Moon occurs once every three years, or 33 months. The last Blue Moon to occur in the United States was in August 2012. The Next Blue Moon will be in July 2015. As a final note, because February has 28 days, it is impossible for a Blue Moon to occur during this month.
The origins of the term “Blue Moon” remain unclear. Because of this, the Blue Moon also refers to another irregularity that can occur, where the Moon appears blue colored in the night sky. Such an event is incredibly rare, and is caused by high altitude dust particles refracting light in such a way to make the Moon appear blue from Earth. Most sources credit this unusual event, occurring only "once in a blue moon," as the true progenitor of the colorful phrase.