"Once In a Blue Moon"- What does it mean?

A neat description of "not very often," it refers to the second full moon within a month - a rare thing indeed.
Full moons happen about every 29.5 days, and since a typical month runs between 30 to 31 days, the likelihood of two in a month is slim.
But over the course of a century there'll be 41 months with two full moons, so once in a blue moon really means - if you want to get literal - once every 2.4 years...

http://www.neatorama.com/2008/12/08/the-science-behind-some-popular-phrases/