In astronomical terms, a blue moon really doesn't denote that long a time span. In fact, a blue moon happens once every 2.7 years on average.
Still, it's a special event that, at least using its modern definition, happens when there are two full moons in a single month.
Today (Aug. 1) we'll see a full-moon and that is a prelude to the blue moon of Aug. 31. As Space.com reports, the Aug. 31 moon will reach its full phase during the day.
"Night sky observers won't have a chance to see two full moons in a single month again until July 2015," says Space.com.
The Washington Post's Capital Weather Gang gives us some blue moon trivia:
"In 1999, you may recall we had two full moons in January and March. And February had no full moons at all.
"In 2018, we'll enjoy double blue moons again in January and March – and once again February is devoid of official lunar plump. Odenwald – the author of 'The Astronomy Café,' 'Back to The Astronomy Café,' and 'Patterns in the Void' – explains on his website that the lunar month is 29.53 days long. The largest number of days in February is 29 days, so February will never see a blue moon."
Earthsky.org, by the way, has a great explanation on what a blue moon used to be and why it changed. In short, the Farmer's Almanac used to define a blue moon as the third full moon of a four-full-moon season. But a 1946 Sky and Telescope magazine piece simplified it as the second full moon in a calendar month and that stuck.
If you go by the old-school definition, however, you'd have to wait until August of 2013 for a blue moon.
(Oh and the moon won't actually be blue.)