E.g. a single character to unambiguously denote weekday, window layout (全／広／高）, and others. Renders everywhere as long as you have a decent CJK font. Bonus: looks a lot less stupid than emojis.
My desktop terminal prompt right now shows [金 19:58:22] - just a single character and as long as it's < 7d ago it's unambiguous what time and day a command exited in case I forget.
You're a bit cheating there :) It's one character that takes up 2 columns.
You could get away with using the first 2 characters from the English names as they are unique. But I know that in English that's unusual but for example in German, the 2 letter abbreviation are the standard ones.
You say weekday, but Google translate and Wikipedia say that 金 means gold?
The days of the week were named after the seven classical planets of astrology (by the Romans, who in turn named the seven classical planets, and hence the days, after Roman deities).
The names were taken up by speakers of Germanic languages, and eventually underwent semantic loan translation: Germanic deities were substituted for the Roman ones (with the exception of Saturday). Some people say that the Germanic deities themselves were culturally identified as one and the same as the Roman deities, but we probably won't ever know for sure (interpretatio germanica, analogously to the interpretatio graeca, the identification of Egyptian, Roman and Greek deities [1]).
And now, a minor correction: The day of Venus, Friday bears the name of the Germanic goddess Frigg, who is not the same as the Norse goddess Freyja. The meeting of the two characters, in a debate against Loki, is described in the Poetic Edda [2].
[1] https://en.wikipedia.org/wiki/Interpretatio_germanica [2] https://en.wikipedia.org/wiki/Lokasenna
"金" would be short for "金曜日", a common way to abbreviate weekday names in calendars etc and yes, Friday is indeed the day of gold - I like to imagine it's the old pay day for workers maybe? :D