Once in a Blue Moon

• Once in a Blue Moon is a phrase used to mean a very rare or unusual occurrence.
• It is based on the occurence of a Blue Moon.

Blue Moon

• Every three years there are two Full Moons in one calendar month.
• In modern times, the second Full Moon has become known as a ‘Blue Moon’.
• The older meaning of ‘once in a Blue Moon’, first used in the 1500’s, derives from the rare occurrence of a blue tinged Moon due to high altitude dust.