This article is from the alt.usage.english FAQ, by Mark Israel firstname.lastname@example.org with numerous contributions by others.
(notes by Philip Hiscock)
The phrase "blue moon" has been around a long time, well over 400
years, but during that time its meaning has shifted around a lot. I
have counted six different meanings which have been carried by the
term, and at least four of them are still current today.
The earliest uses of the term are in a phrase remarkably like
early references to "green cheese". Both were used as examples
of obvious absurdities about which there could be no argument. Four
hundred years ago, if someone said "He would argue the moon was
blue", the average 16th-centuryman would take it the way we take
"He'd argue that black is white." The earliest citation is a 1528
poem "Rede Me and Be Not Wroth": "Yf they say the mone is blewe/We
must believe that it is true."
This understanding of a blue moon's being absurd (the first
meaning) led eventually to a second meaning, that of "never". To
say that something would happen when the moon turned blue was like
saying that it would happen on Tib's Eve (at least before Tib got a
day near Christmas assigned to her).
But of course, there are examples of the moon's actually turning
blue; that's the third meaning: the moon's visually appearing blue.
When the Indonesian volcano Krakatoa exploded in 1883, its dust
turned sunsets green and the moon blue all around the world for the
best part of two years. In 1927, a late monsoon in India set up
conditions for a blue moon. And the moon here in Newfoundland was
turned blue in September 1950 when huge forest fires in Alberta
threw smoke particles up into the sky. Even by the 19th century, it
was clear that although visually blue moons were rare, they did
happen from time to time. So the phrase "once in a blue moon" came
about. It meant then exactly what it means today: that an event
was fairly infrequent, but not quite regular enough to pinpoint.
That's meaning number four, and today it is still the main one.
I know of six songs which use "blue moon" as a symbol of sadness
and loneliness. In half of them, the poor crooner's moon turns to
gold when he gets his love at the end of the song. That's meaning
number five: check your old Elvis Presley or Bill Monroe records
for more information.
Finally, in the 1980s, a sixth meaning was popularized (chiefly
by the game Trivial Pursuit): the second full moon in a month. The
earliest reference cited for this is The Maine Farmers' Almanac for
1937. Rumour has it that when there were two full moons in a
calendar month, calendars would put the first in red, the second in