We are fast approaching a blue moon. The event will occur once in a blue moon and the date happens to be August 31, 2012. It's not that the moon will appear blue, however, it will be the second full moon occuring within the same month.
It's exciting to enjoy this rare occasion, since a blue moon only takes place about every two and a half years. When the first full moon takes place at the beginning of the month there is time for a second. Some also refer to a blue moon as the second full moon within the same season. Typically there are 12 full moons each year. One per month.
Yet in each solar calendar year there are about eleven days more than the lunar year of 12. The extra days add up so every two to three years we get to enjoy a blue moon.
The Older definition of a Blue Moon: Normally an astronomic season will have three full moons. When a fourth full moon occurs during a season, it is a Blue Moon With this definition the next Blue Moon will be August 20th 2013 since there are four full moons between the June 21, 2013 solstice and Sept 22, 2013 equinox. The more familiar definition of two full moons during a calendar month dates back to a free lance writer in the mid 1900's. He mistakenly interpreted the original definition to the second full moon in a calendar month. This definition has permeated our Western Culture.