Once in Blue Moon

 Something fairly rare is going to occur on October 31, Halloween night, this year. 

There is going to be a "Blue Moon."

This is the name for a second full moon in one calendar month. 

But this year, 2020, the Blue Moon is even more unique. 

A Blue Moon only happens on Halloween about every 19-20 years and a Blue Moon that is visible in every time zone only happens every several decades!! The last time this occurred- blue moon on Halloween visible in every time zone- was in 1944 during WW2. 

So, honestly, what is going to occur is very rare and why we use the idiom "once in a blue moon" to describe something that rarely happens!

If you want the scientific details I've got a quick explanation below from Brittanica's website:

The period from one full moon to another is about 29 1/2  days, so when two occur in the same month, the first of these full moons is always on the first or second day of the month. February, which has only 28 days (29 days in leap years), can never have a blue moon. Months with 31 days—January, March, May, July, August, October, and December—have a much better chance of hosting a blue moon because of their length. On average, a blue moon occurs once every 33 months or full moons, 41 times per century, or about seven times every 19 years. An even rarer event is when two blue moons happen in the same calendar year, which happens about four times per century.

I hope you have clear skies wherever you are and can see this beautiful occurrence.