Today is the day for any unlikely event you care to see come to pass because tonight will be a genuine blue moon. A blue moon is the second full moon occurring in a calendar month, and that’s what you’ll get (weather permitting) tonight. The origin of the term blue moon is a little less clear. According to Wikipedia (oh dispenser of undisputed truth):
“The suggestion has been made that the term "blue moon" for "intercalary month" arose by folk etymology, the "blue" replacing the no-longer-understood belewe "to betray". The original meaning would then have been "betrayer moon", referring to a full moon which would "normally" (in non-intercalating years) be the full moon of spring, while in intercalating year, it was "traitorous" in the sense that people would have had to continue fasting for another month in accordance with the season of Lent.”
This explanation seemed a little contrived for me, though. I mean a lot of the English language can trace its origin back to biblical references, but Lent is a once-a-year thing and to have a moon named after something that's rare at best doesn't sit well with me. Digging into the Old Farmer’s Almanac confirmes the belewe reference, but also gives a second possible origin of the term:
“…in the March 1999 issue of Sky & Telescope magazine, author Phillip Hiscock revealed one somewhat confusing origin of this term. It seems that the modern custom of naming the second full Moon of a month "blue," came from an article published in the March 1946 Sky & Telescope magazine. The article was "Once in a Blue Moon," written by James Hugh Pruett. In this article, Pruett interpreted what he read in a publication known as the Maine Farmers' Almanac (no relation to this Farmers' Almanac, published in Lewiston, Maine), and declared that a second full Moon in a calendar month is a "Blue Moon."
However, after reviewing the Maine Farmer's Almanac, Hiscock found that during the editorship of Henry Porter Trefethen (1932 to 1957), the Maine Farmers' Almanac made occasional reference to a Blue Moon, but derived it from a completely different (and rather convoluted) seasonal rule. As simply as can be described, according to Trefethen's almanac, there are normally three full Moons for each season of the year. But when a particular season ends up containing four full Moons, then the third of that season is called a Blue Moon! To make matters more confusing, the beginning of the seasons listed in Trefethen's almanac were fixed. A fictitious or dynamical mean Sun produced four seasons of equal length with dates which differed slightly from more conventional calculations. So, basically the current use of "Blue Moon" to mean the second full Moon in a month can be traced to a 55-year-old mistake in Sky & Telescope magazine.”
Regardless of which origin you believe, the term blue moon has influenced everything from beer to music to literature. It has a kind of poetry, a wistfulness and quiet desperation that nests deep inside so much good literature. A current favorite of mine is the poem The Blue Moon by James Francis Carlin MacDonnell.
The Blue Moon
Memory is as blue
As the small flax-flower’s dew,
The twilight’s distant skies
And your far eyes:
Blue as the meadows seen
In reality as green;
Blue as the broad moon-light
That is really white.
Memory is as blue
As the world that relates to you,
From the heavens over all,
To your blue shawl:
Blue as the roads that may
Once more be dusty grey,
For one whose sight of mind
Memory as blue
As the winds that sally through
The dark blue shadows, deep
In your blue sleep:
Blue as your lips, to be
That red reality,
Which I shall meet when the light
Of the moon is white.
So, get out there under that old blue moon with the one you love and make the most of this August night!