A Blue moon is a name given to an irregularly timed full moon. Most years have twelve full moons which occur approximately monthly, but each calendar year contains those twelve full lunar cycles plus about eleven days to spare. The extra days accumulate, so that every two or three years there is an extra full moon (this happens every 2.72 years). (Source: Wikipedia)
So, its frequency of occurrence would be:
1/ 2.72 * 365 * 24 * 60 * 60) = 1.16580118 × 10 ^ -8
Now, this one is good. Excellent humor!
But don't get how they arrived at that number. Average considering the leap year does not result in that either.