A rare 'blue moon' will grace the night sky today (31-08-2012). When a full moon appears in the night sky for the second time in a calendar month, the occurrence is known as a 'Blue Moon'. This month, the first full moon was on 2nd August we can see it for the second time today night at 19:28 PM.
Despite its colorful name, there's nothing particularly blue about a blue moon. It simply means the second full moon of the month, so don't expect to see a different hue this night.
The last 'blue moon' was sighted in December 2009 and the next will occur on July 31, 2015.
As per wikipedia :
A blue moon can refer to either the third full moon in a season with four full moons, or the second full moon in a month. Most years have twelve full moons that occur approximately monthly. In addition to those twelve full lunar cycles, each solar calendar year contains roughly eleven days more than the lunar year of 12 lunations. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. Lunisolar calendars have rules about when to insert such an intercalary or embolismic ("leap") month, and what name it is given;e.g. in the Hebrew calendar the month Adar is duplicated. The term "blue moon" comes from folklore. Different traditions and conventions place the extra "blue" full moon at different times in the year. In the Hindu calendar, this extra month is called 'Adhik(extra) masa (month)'. It is also known as purushottam maas, so as to give it a devotional name.
31 December 2009 Blue Moon with partial lunar eclipse
Source : http://en.wikipedia.org/wiki/Blue_moon