The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every 2-3 years, so these sorts of Blue Moons come about that often.
http://earthsky.org/space/when-is-the-next-blue-moon
The next Blue Moon will be on May 21, 2016. It’s a seasonal Blue Moon, the third of four full moons between the March equinox and the June solstice of 2016. Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two.
http://earthsky.org/space/when-is-the-next-blue-moon
The next Blue Moon will be on May 21, 2016. It’s a seasonal Blue Moon, the third of four full moons between the March equinox and the June solstice of 2016. Seven times in 19 years there were — and still are — 13 full moons in a year.
http://earthsky.org/space/when-is-the-next-blue-moon
According to modern folklore, a Blue Moon is the second full moon in a calendar month. Usually months have only one full moon, but occasionally a second one sneaks in. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month.
http://science.nasa.gov/science-news/science-at-nasa/2004/07jul_bluemoon/
The modern day definition of a Blue Moon is when there are 2 full moons in one Month. A full moon occurs roughly every 29.5 days and on the rare occasions when the full moon falls at the very beginning of a month there is a good chance a Blue Moon will occur at the end of the month.
http://www.moongiant.com/Blue_Moon_Calendar.php
So the July 31 full moon was called a Blue Moon, according to this definition. The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month.
http://earthsky.org/space/when-is-the-next-blue-moon
The next year of double monthly blue moons is coming up in January and March, 2018 – and then, after that, in January and March, 2037. Very rarely, a monthly Blue Moon (second of two full moons in one calendar month) and a seasonal Blue Moon (third of four full moons in one season) can occur in the same calendar year.
http://earthsky.org/space/when-is-the-next-blue-moon
The first full moon was spotted on 1 July and the second will appear on the last day of the month – Friday, July 31. This appearance is the first such occurrence in three years since the last blue moon month in August 2012, the next will not be until January 2018.
http://www.telegraph.co.uk/news/science/space/11775016/Rare-super-Blue-Moon-to-light-up-summer-night-sky.html
The error led to the widely accepted definition of blue moon today: the second full moon in a given month. A blue moon occurs roughly once every 2.7 years. The next blue moon, according to this folklore, will be July 31, 2015, following a full moon on July 1, 2015.
http://www.space.com/15455-blue-moon.html
According to the more recent definition, a blue moon is the second full moon in a calendar month. For a blue moon to occur, the first of the full moons must appear at or near the beginning of the month so that the second will fall within the same month (the average span between two moons is 29.5 days).
http://www.infoplease.com/spot/bluemoon1.html
When is the Next Blue Moon
The Next Blue Moon is on May 21, 2016. The next blue moon was on May 21, 2016.