A "Blue moon" is an occasion of no organic significance, generated by the correspondence between lunar Earth-orbit and a modern calendar.
It is, technically, the second full moon in a month.
The average lunar orbit is 29.5 days. The full moon, or any given lunar phase, will occasionally happen twice within the arbitrary period of the modern calendar month.
The non-phenomenon occurs about 7 times every 19 years "once in a blue moon."
Two folks who were new friends of mine in Kilkenny Ireland were shooting pool at Syd's pub.
It was a "blue moon," the second full moon in the month.
Syd's pool room is a small, mildly lighted space up a few steps beyond the toilets and cigarette machine at the back of the establishment.
Mick and Molly, both of whom play brilliant pool, were near the end of a game in fact it was the endgame; down to the black. Molly was shooting at the black. She missed.
Rare enough. In a fairly straighforward shot, she just missed the black, by the finest whisper. It was minutely close, but it did not hit that much was sure, from where I stood. Two shots to Mick a significant development in the game. The trouble was that Molly didn't see it that way couldn't or wouldn't.
But I'd seen it, with my eyes, and had no reason to believe one way or the other. I saw the miss no movement of the black, as plain as could be. So I called it like that.
Faced with dispute, I stuck with it, too. I didn't argue, but I insisted. The black had gone untouched.
Molly stormed out. She left. Gone, argument finished.
Later, Mick and I crossed Rose Inn Street for a cup of tea. Outside, that full moon shone on clouds in two layers, bulbous higher cumuluous and a radiant sheen of lower, thin whispy sheets moving on two different wind headings.
Mick and I talked about what had happened, but we did not figure it out.