Once in a blue moon
On the almanac page for August 1937, the calendrical meaning for the term "blue moon" was given.
That explanation said that the moon "... usually comes full twelve times in a year, three times for each season."
Occasionally, however, there will come a year when there are 13 full moons during a year, not the usual 12. The almanac explanation continued:
"This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar of thirteen months for that year, and it upset the regular arrangement of church festivals. For this reason thirteen came to be considered an unlucky number."
And with that extra full moon, it also meant that one of the four seasons would contain four full moons instead of the usual three.
"There are seven Blue Moons in a Lunar Cycle of nineteen years," continued the almanac, ending on the comment that, "In olden times the almanac makers had much difficulty calculating the occurrence of the Blue Moon and this uncertainty gave rise to the expression 'Once in a Blue Moon.'"
Not sure i buy the reasoning that 13 became unlucky because it upset the "regular arrangement of church festivals," but there may be some evidence that the extra 13th moon is at the root of the number 13's significance. But let's leave that aside for the moment. The article goes on to precisely note the introduction into the culture of the now common belief that the blue moon is, in fact, the second full moon of a calendar month. It cites specifically a radio broadcast of a program called Star Date, from January 31, 1980, which in turn cited another Sky & Telescope article from 1946 misinterpreting the previous article referenced above.
I remember that Star Date episode - i was volunteering at the time at a college radio station that aired the program twice daily. And i pocketed that little tidbit of knowledge. And somehow i just assumed that it was something that calendar makers had been doing since the days of the Julian calendar or so, that is, denoting the occasional second full moon of a month by showing it in blue.
But that is apparently not the case. And so, in a season of 4 full moons, which happens 7 times every 19 years, the third full moon is the one which should receive the designation "blue." And since the September full moon occurred after the equinox, and the December full moon will occur slightly before the winter solstice, that makes the November full moon the third of this autumn.
Color it blue.