If something doesn’t happen very often, you might say that it happens once in a blue moon English. What about in other languages?
The phrase once in a blue moon first appeared in first around 1821 and refers to something that happens very rarely, very infrequently or almost never. An alternative version is apparently once in a purple moon [source].
A blue moon is a second full moon during a calendar month, which happens once every 32 months. A blue moon can also refer to a full moon that appears to be blue and bigger than usual [source].
In (Mandarin) Chinese, the equivalent is 千载难逢 [千載難逢] (qiānzǎinánféng), which means “an opportunity difficult to come by even in a thousand years”. This expression comes from folk tales, in which such rare opportunities might lead to success or life-changing events [source].
In Czech, the equivalent is jednou za uherský rok , which means ‘once in a Hungarian year’. Apparently this idiom dates from the Turkish wars, when Czech soldiers were hired in Hungary for a certain period of time, which was constantly extended [source]. The Slovak equivalent, raz za uhorský rok (“once a Hungarian year”), probably comes from the same roots.
In Polish, they say raz na ruski rok (“once in a Ruthenian year”), which refers to the fact that the Gregorian calendar used by East Slavs has longer months [source]. Alternatively, they say od wielkiego dzwonu (“from the big bell”), which refers to The Sigismund Bell, the largest of the five bells in the Sigismund Tower of Wawel Cathedral in Kraków, which is used only on special occasions [source].
In French, you might say tous les trente-six du mois (“every 36th of the month”) or une fois toutes les lunes (“once every moon”) [source].
In German, you could say alle Jubeljahre (“every jubilee year”), which refers to biblical jubliees that come round every 50 years [source].
In Russian, they say раз в сто лет (“once every hundred years”) [source].
In Spanish, they say cada muerte de obispo (“every death of a bishop”), or de Pascuas a Ramos (“from Easter (Day) til Palm Sunday”) – Palm Sunday comes before Easter Day [source].
In Welsh, one equivalent is unwaith yn y pedwar amser (“once in the four seasons”) and another is unwaith yn y pedwar gwynt (“once in the four winds”) [source].
Are there interesting similar idioms in other languages?
