Just in case ya'll don't know, this August we are actually gonna have two full moons. The first will be tomorrow and the second will be on the 31st. (Oh and the title IS a reference to Pocahontas :) for those of ya'll who didn't catch it)
When two full moons happen within the same month, the second is usually called a "blue moon", even though the moon usually looks like an other. Another interpretation of the blue moon rule concerns when four full moons occur within the same season (spring, summer, fall, or winter) and the third full moon out of the four is considered the blue moon. Most years have twelve full moons and they normally occur monthly. However, the solar calendar year has about eleven more days than the lunar year. Those extra days accumulate and so every 2~3 years (2.66 to be exact) there is an extra full moon. It's works very much like leap years, which is why some people use the term leap month rather than blue moon.
The term "once in a full moon" has been used since the early 1800s and refers to situations that are uncommon, but not rare. The ironic part though, is that blue moons are actually not that uncommon. Like I said before, they occur about every three years. In 1999 there were two blue moons within three months of each other. The association with blue moons and rare occurrences may come from the literal meaning of "blue moon" where the full moon appears to have a blue tint, which actually is a rare event. This event can be caused by smoke and dust particles in the atmosphere, which happens after severe forest fires or volcanic eruptions.