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Blue Moon

The saying "Once in a blue moon" refers to something that happens very rarely. And a rare Blue Moon is happening tomorrow night! So why is a full moon called a Blue Moon? There are two different times when the term is used and neither have anything to do with the moon's color. Both do come from the saying though, that according to folklore has been around for over 400 years!

Blue Moon

The first reason a full moon is referred to as Blue Moon is when two full moons occur in the same month.  The last full moon, the Full Harvest Moon, was on October 1st so tomorrow night's full moon, the second this month, is a monthly Blue Moon.  The last Blue Moon named for this reason was on March 31, 2018.  So you can see that this is a rather rare occurrence!  

Also rare, is the other reason a moon is called a Blue Moon.  It is the name given to the third full moon when there are four full moons in the same season.  The last seasonal Blue Moon was on May 18, 2019.

By either definition seeing a Blue Moon on Halloween only happens about every 19 years making tomorrow night's a sight we haven't seen in many years and won't see again until 2039. Enjoy this once in a blue moon occurrence!