Blue Moon

Blue Moon or Ice Moon is seen during a lunar eclipse, when the infrared rays emitted by the Earth to the Moon is not sufficient in times of extreme cold, hence the name Moon ice cream.
The Moon is at this time illuminated by ultraviolet light emitted by the Earth which gives it a slight blue tint. Fine volcanic dust circulating in the Earth's atmosphere can also give a slight blue tint to the moon. But Blue Moon comes from the English term "blue moon" which designates currently the second full moon of the month, relatively frequent phenomenon since it takes a year of 13 full moons instead of 12 normally.
Usually there are no more than one full moon per month, this shift is due to the fact that the lunation lasts 29.53 days while our last 30 calendar months or 31 days.
The Blue Moon occurred Friday, August 31, 2012 as the first full moon of the month took place Thursday, August 2, 2012.
The previous agreement occurred on 31 December 2009 and the next will occur July 31, 2015 where we will once again attend two full moons in the same month.
When there is no full moon in February, it can happen since the month of February is shorter than the lunar cycle, it is possible that the year has still, 13 full Moons.
This means that there will be two moons in year but this is relatively rare, three to four times a century, this was the case in 1961, 1999 and this will be the case in 2018. However, the term "blue moon" was in past times several different interpretations. It appointed the thirteenth moon of the year or the third moon in a season that had four.