Once in a blue moon
The first question to ask on examining this expression is simple; what is a blue moon anyway? And why call it blue?
The first part is not too much trouble to answer. A blue moon is very simply the second full moon to occur in a calendar month. Not something we see very frequently.
So why do we call it blue? Perhaps because it sounded absurd and added to the sheer unlikelihood of something happening. However, although of course the moon doesn’t turn blue if it happens to be a full moon twice in one calendar month, there are some things that can make the moon appear blue.
Massive volcanic eruptions can throw dust or smoke high up into the Earth’s atmosphere, and as the moonlight passes through the tiny dust or smoke particles, it can appear bluish. Fortunately, these volcanic eruptions are also very rare, which only adds to the effectiveness of the expression!
There are some lovely expressions in other languages about rare or unlikely events. Here are just a few. Welsh has unwaith yn y pedwar amser, which means “once in the four seasons”, or once a year, if you like. German is a little less optimistic, with Das kommt alle Jubeljahre einmal vor, translating as “it comes around in the Jubilee year”. You can also use Schaltjahr instead of Jubeljahre, in which case it would be “leap year”.
I have to admire the gloriously humorous expressions from Spanish and Italian; cada muerte de obispo; “every time a bishop dies” from Spanish, and going higher up in authority; ogni morte di papa; “every time a pope dies” from Italian. Just wonderful!
From Slovenia comes a lovely expression which really implies that it will never happen, even if it could, a feeling I am sure we are all familiar with; ob svetem nikoli, which means “On Saint Never’s.”
Finally, in Greek there are two expressions that spring to mind. The first is του αγίου ποτέ, (tou ayiou pote), which means “on Saint Never’s Day”, just as the Slovenian expression does. The second expression is στη χάση και στη φέξη (sti hasi kai sti fexi). Although the word φέξη is no longer used outside of this expression, its meaning is still clear. It is the only expression outside of English on my list to refer to the moon. It means “at the waxing and waning”.
Many thanks, as ever, to @KnightGhost for her help with the Slovenian