The term “Blue Moon” has nothing at all to do with colour. It is commonly accepted that the term refers to a full moon that rises twice in one month. This occurs around once every 33 months. As the cycle of the moon is 29.5 days, and as the number of days in a month is greater, we see this occurrence only rarely. Hence the term “Once in a blue moon”

The next “blue moon” will be in January and March of 2018blue moon