Surprisingly, it's not a very old expression. Philip Hiscock, in his fascinating article about the folklore of the Blue Moon, says that it was first used about 150 years ago as a way of saying not very often.
It wasn't used in the astronomical sense of these web pages until as recently as 1946.
The expression Blue Moon, however, dates back to at least the time of Shakespeare.
Copyright © 1995-2006 by David Harper and Lynne Marie Stockman