On December 31, 2009, we will have the second full moon in the same month - known as a blue moon. It happens every 41 months or sometimes more or sometimes less. So "once in a blue moon" actually means "once every two-and-a-half years" give or take.
The next blue moons until 2020 are:
2012 on August 31
2015 on July 31
2018 on January 31
2020 on October 31

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