This is quite possibly the most famous yojijukugo in Japan. I’m not entirely sure how it took me so long to get around to it.
Literally: one – time (period) – one – meeting
Alternately: (Every time you meet someone, you should treat it as) a cherished once-in-a-lifetime encounter. Every time you meet someone is precious (because it might be your last). Focus on the moment with an open and honest heart.
Notes: 一期 is “one lifetime”; 一会 is “one encounter.” The nonstandard readings, even among the set of available Chinese-derived pronunciations (期 is often read ki, and 会 is often kai), mark this phrase’s roots in Buddhist terminology and thought – but it doesn’t come to us from an ascetic monk. Rather, this four-character compound is said to come from a saying by one of the disciples of the tea-ceremony master known as Sen no Rikyuu, during the late 16th century. I could go on – this phrase unfolds and unfolds and unfolds into so many areas of Japanese history and culture – but perhaps the rest should be left as an exercise for the reader.
This yojijukugo‘s become a commercial product too. Picture almost unrelated.