Once In A Blue Moon

An explanation of the phrase 'Once In A Blue Moon' has been posted at Reader's Digest: "A blue moon is defined as a second full moon within a calendar month. Since a lunar cycle lasts 29.5 days and a calendar month 30.5 days, it occurs only once every two and a half years or so. Astronomers have borrowed the phrase "blue moon" to describe this phenomenon from the traditional expression meaning rarely or never. A second full moon in a month is not actually blue in colour, yet there have been a number of occasions when the moon did appear to be blue. Dust from the Indonesian volcano Krakatoa turned the moon blue in 1883, as did smoke from western Canadian forest fires in 1951." More facts here.