New Year's eve will present us with a blue moon. I had no idea until my husband and daughter started talking about it tonight at the dinner table. A blue moon is the 13th full moon in one calendar year - or the second full moon in a calendar month. It only occurs "once in a blue moon." I say that, not to be funny, but to illustrate the way an idiomatic expression can become part of our normal understanding without us knowing its origin. Everyone knows that "once in a blue moon" indicates a rare event. I have often used that expression but had no idea why it had that meaning. So around the dinner table, the light bulb turned on and I will use this expression from now on with a much deeper appreciation for its origin. BTW, I did go to Wikipedia to verify the meaning.
But back to the blue moon...it only occurs about once every 2.7 years, but a blue moon on New Year's Eve is even rarer, only occurring about every 20 years. So we're in for a treat. What a fun way to usher in the year 2010!
Just curious...did you already know where this expression came from?