I can't comment on what definition of blue moon they use, but Google calculator says that ~0.37 occur every year: http://www.google.com/search?hl=en&q=1+year+*

I can't comment on what definition of blue moon they use, but Google calculator says that ~0.37 occur every year: http://www.google.com/search?hl=en&q=1+year+*+once+in+a+blue+moon http://www.google.com/search?hl=en&q=1+year+*+once+in+a+...