The expression 'Once in a Blue Moon' is used to express a rare and often special event. This is because a Blue Moon occurs just once in every two and half years or so. This special Moon occurs when a second full Moon falls in any one calendar month. Usually months have only one full Moon, but occasionally there can be a second one - named a Blue Moon.

Full Moons occur every 29 days, but most months are 30 or 31 days long; so it is possible to fit two full Moons in a single month. This happen...


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