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). July 2004 will have two full moons: the first on July 2, the second on July 31—that second full moon is called the blue moonthis certainly is not a common occurence - that explains the term "once in a blue moon". the next time a calendar month is going to have two full moon nights will be in june 2007.
there is one more definition, this one is a little more complicated (again quoting from infoplease)
the blue moon is the third full moon in a season that has four full moons ....This is referred to in the HIndu calendar as an 'adhik maas' (additional month) - actually i am guessing that the adhik maas refers to this. i will try to confirm this info. but nevertheless, this is quite rare too. it will occur in august 2005, and then only in may 2008.
... It has to do with the Christian ecclesiastical calendar. Some years have an extra full moon—thirteen instead of twelve. Since the identity of the moons was important in the ecclesiastical calendar (the Paschal Moon, for example, used to be crucial for determining the date of Easter), a year with a thirteenth moon skewed the calendar, since there were names for only twelve moons. By identifying the extra, thirteenth moon as a blue moon, the ecclesiastical calendar was able to stay on track.
yet, this does not seem to be the origin of the "blue moon" phrase. According to the Oxford English Dictionary, the first reference to a blue moon comes from a proverb recorded in 1528:
If they say the moon is blue, We must believe that it is true.
i am not sure about that but check this out: the month of feb has 28 days, while the moon cycle is 29.5 - all other calendar months have a longer duration. so it is quite likely for both jan and march to have two full moons (double blue moon in a year!) and february get totally left out - and have no full moon at all .... that is going to occur next only in 2018, and then in 2037 - truly a once in a blue moon