The phrase “once in a blue moon” means extremely infrequently, so rarely as to be almost tantamount to never.
From literary evidence the unusual tinge to the face of the moon which led someone to call it a “blue moon” was not observed until after the middle of the last century; nevertheless it is highly probable that this phenomenon had been observed by mariners some centuries earlier, but, like many other notions and expressions long familiar to seafaring men, it did not come to the notice of writers for many, many years.
But, with another thought in mind, as long ago as 1528 a rimester published these lines:
Yf they saye the mone is belewe,
We must beleve that it is true.
Then the next year “green cheese” entered the picture in the lines of another writer: “They woulde make men beleue . . . that ye Moone is made of grene cheese.”
Apparently, then, there were two schools of thought back in the early sixteenth century, one maintaining that “ye Moone” was made of “grene” cheese, and the other stoutly affirming that it was “belewe.”
Actually these ancient humorists were just punsters with a taste for metaphor; for by “green cheese,” it was not the color but the freshness that was referred to, the moon, when full and just rising, resembling both in color and shape a newly pressed cheese.
By “blue cheese” the ancient reference was to a cheese that had become blue with mold, metaphorically transferred, probably, to the comparatively rare appearance of the moon on unusually clear nights when the entire surface of the moon is visible although no more than a thin edge is illuminated.
Thus, our phrase, “once in a blue moon” may actually date back to the sixteenth-century saying that “the mone is belewe.”