Tomorrow, May 2nd is the full moon. We have another full moon at the end of May on the 31st. This is known as a blue moon. Here are a few blue moon facts.






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.....A Full Moon on Halloween must also be a Blue Moon, and it happens five or six times during the 21st century....................

Farmer's Almanac blue moons
The older meaning of blue moon to name an extra full moon, as was used in the Maine Farmer's Almanac, was the third full moon in a quarter of the year when there were four full moons. (Normally a quarter year has three full moons.) The division of the year into quarters for this purpose has the dividing line set between March 21 and March 22. This has to do with the rule for setting the date for the Christian Holy Day of Easter, which depends on the last full moon (as calculated by a somewhat inaccurate formula) on or before the Equinox on March 21 (which is also somewhat inaccurate).
This meaning of blue moon was lost when the editors of the original Farmer's Almanac died. It was recovered only when researchers for Sky & Telescope magazine noticed that the Maine Farmer's Almanac from 1829 to 1937 reported blue moons that did not fit the meaning of the term calendar blue moon described below.

Visibly blue moon
The most obvious meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, such as happened after forest fires in Sweden in 1950 and Canada in 1951 and, notably, after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years.

Earliest use in English
The origin of the term Blue Moon is steeped in folklore, and its meaning has changed and acquired new and interesting meanings and nuances over time. The earliest known recorded usage was in 1528, in a pamphlet entitled Rede Me and Be Not Wrothe: "Yf they say the mone is belewe / We must beleve that it is true" [If they say the moon is blue, we must believe that it is true]. This implies the expression had a meaning of something that was absurd, and bears close resemblance to another moon-related adage first recorded in the following year: "They would make men beleue ... that ye Moone is made of grene chese". [They would make men believe ... that the moon is made of green cheese]

How often does a year have two Blue Moons? Four or five times in a hundred years
Which months have the most Blue Moons? As you might expect, the 31-day months are the winners. October, August and July take the top medals Once in a Blue Moon
Once in a Blue Moon ... is a common way of saying not very often, but what exactly is a Blue Moon?
According to the popular definition, it is the second Full Moon to occur in a single calendar month.
The average interval between Full Moons is about 29.5 days, whilst the length of an average month is roughly 30.5 days. This makes it very unlikely that any given month will contain two Full Moons, though it does sometimes happen.
On average, there will be 41 months that have two Full Moons in every century, so you could say that once in a Blue Moon actually means once every two-and-a-half years.