Once in a Blue Moon,” is a common way of saying. ‘Not very often,’ but what exactly is a Blue Moon? It is the second Full Moon to occur in a single calendar month. In 2007, there is a Blue Moon on June 30 in India.
The next ‘Blue Moon’ will be the second full moon on December 31, 2009.

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.

In India the first Full Moon occurred on June 1.

The  average  interval  between  two full  moons is about 29.5 days, whilst the duration of an average calendar month is roughly 30.5 days.

This makes it very unlikely that any given month will have two Full Moons, though it does sometimes happen.
Does the Moon actually change colour during a Blue Moon? No, but there have been rare occasions when the Moon really did turn blue due to some weather phenomenon like dust in the atmosphere.

The phrase “Once in a Blue Moon” has been a part of the English language for about four hundred years. During that time, it’s had half a dozen different meanings.

The phrase “Once in a Blue Moon” appears to have been a colloquial expression long before it developed its astronomical sense. 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.”

Does a Blue Moon have any effect on our life or behaviour? It has no more effect than any other Full Moon.
Which months have the most Blue Moons? As you might expect, the 31-day months are the likely to have more Blue Moons. October, August and July take the cake.

Can a month have no Full Moon? Sometimes, February misses out on a Full Moon, but it’s the only month that can have no Full Moon, because except the leap years it has only 28 days.

What about two New Moons? They also occur every two or three years, just like two Full Moons, but fewer people notice them. Watch the Blue Moon on June 30, unless it is cloudy.