Re: Frequency of Blue Moons

"Blue moon" is a nickname for the second full moon in a month.
Full moons happen about every 29.53 days (the precise average period is 29.53059 days,
but it's not always that exact interval because their are variations through the year,
as the Earth-moon distance, Earth-sun distance, and sun-moon distance change)
So you'll get a blue moon in a 31-day month if and only if the first full moon 
occurs less than 1.47 days into the month, which'll happen, on average, 1.47 / 31
of the time... or about 5% of 31-day months.
Similarly, in a 30-day month you get a blue moon every .47/30, or 1.5%.
These chances work out to a blue moon every 19 months, on average.  It varies
widely, however, and you can have several years without a blue moon at times.
You can't ever have a blue moon in February, since at most it has 29 days.

Hope this helps; let me know if you've got any further questions!

-Jim Moskowitz
The Franklin Institute
jimmosk@fi.edu