http://www.bbc.co.uk/newsbeat/article/33731279/its-a-blue-moon-tonight-heres-what-that-means

A “Blue Moon” is defined as the 2nd of 2 Full Moons that appear within the same month. They are surprisingly rare (the last was in 2012, and the next is in 2018). Skies are clear here in Nottinghamshire, so I hope that it is the same for you.

PS
“Blue” means ‘rare’. They are rarely bluish.

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Alex Kemp