Over the next twenty years there will be a total of 17 blue moons, with an almost equal number of both types of blue moons occurring. No blue moon of any kind will occur in the years 2006, 2011, 2014, and 2017.

The more recent phenomenon, where the blue moon is considered to be the second full moon in a calendar month, last occurred on Nov. 30, 2001. Two full moons in one month may occur in any month out of the year except for February, which is shorter than the lunar cycle.

The other, older blue moon event, which happens when there are four full moons in a season, last occurred on Nov. 20, 2002. Since this type of blue moon is reckoned according to the seasons, it can only occur in February, May, August, or November, about a month before the equinox or the solstice. Twice in a Blue Moon

The rare phenomenon of two blue moons (using the more recent definition) occurring in the same year happens approximately once every 19 years. 1999 was the last time a blue moon appeared twice, in January and March.
Blue Moons 2004–2010

The months of the double blue moons are almost always January and March. That is because the short month that falls in between them, February, is a key ingredient in this once-every-nineteen-year phenomenon. For January and March to each have two full moons, it's necessary for February to have none at all. Since February is usually 28 days long, and the average span between full moons is 29.5 days, if a full moon occurs at the end of January, it's possible for the next full moon to skip February entirely and fall in the beginning of March. Once in a Blue Moon

"Blue moon" appears to have been a colloquial expression long before it developed its calendrical senses. 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.

Saying the moon was blue was equivalent to saying the moon was made of green (or cream) cheese; it indicated an obvious absurdity. In the 19th century, the phrase until a blue moon developed, meaning "never." The phrase, once in a blue moon today has come to mean "every now and then" or "rarely"—whether it gained that meaning through association with the lunar event remains uncertain.

July 31, 2004
Second full moon in month

August 2005
Third full moon in a season of four full moons

June 2007
Second full moon in month

May 2008
Third full moon in a season of four full moons

Dec. 2009
Second full moon in month

Nov. 2010
Third full moon in a season of four full moons