The expression "once in a blue moon" means a rare event. And indeed a blue moon is a rare event. The definition of a blue moon according to Merriam-Webster is "a second full moon in a calendar month," and that only happens seven times in a 19 year cycle.
As a child I'd heard this expression many times and always wanted to see a blue moon. Blue being my favorite color and the full moon a beautiful celestial event, I could not imagine a prettier site. So the first time a blue moon happened when I was old enough to know, I waited outsidefor the moon to rise. Imagine my confusion when the moon was still a luminous pearly white!
We will be experiencing a "blue moon" this Friday, August 31, 2012. I will have to go check again, just to make sure it is not a cornflower blue. Maybe just in my imagination.
A full moon rises in the night sky over an ancient cottonwood tree twisting in the blue gray fog.