Stock Photo - Blue Moon, Leon province, Spain. They call Blue Moon (Blue Moon English translation) to the second full moon occurring during the same month of the Gregorian calendar (commonly used in the West), which occurs approximately every two, three or four years and, originally, the third full moon when a station any year four full moons occur instead of three. The phenomenon ""Blue Moon"" became popular when it happened twice in 1999 (January-March). The media widely reseñaron the event, little known until then. Naturally, in February 1999 there was no full moon . The origin of the term based on the Gregorian calendar, its use is likely to spread in medieval Europe. The Spanish translation is not quite complete, and that the term comes from the English blue (""blue""), which in turn comes from a corruption of the Old English belewe, which actually means ""traitor"" as an additional moon Spring meant to extend the fasting of Lent. Between three and seven times in every century there are two blue moons in one year. Because February is the only one whose length is less than the lunar cycle, the first always occurs in January and the second, in descending order of probability, in March, April or May. Two blue moons 2 and December 31, 2009 were observed, agreeing that the December 31, 2009 was a partial eclipse of the moon, the full moon was in December. The last blue moon occurred in July 2015, this year.