Wikipedia seems to treat "yoi" as used in the context as interchangeable with shizentai (natural stance). Most karate sites do also translate the meaning as "ready". However, when I try to find the appropriate kanji, the only "yoi" I find is 良, which (as far as I can tell) means good, but has no translation equivalent to "ready" in English.

2015年5月13日 · 良い yoi — 良く yoku. いい ii — よく yoku. When typing いい, IME offers 良い in the lookup table. It makes me wonder whether いい is just an alias of 良い but it is pronounced differently only in the dictionary form. What is the difference between 良い and いい?

2019年10月17日 · When doing karate years ago the sensei would use the word ''yoi'' to instruct the students to take a ready/prepared stance. I didn't speak any Japanese then but I remember the word. Now I'm learning Japanese and I've come across the word ''yoi'' which means good/nice. Are these two meanings for the same word or are they just homonyms?

2021年8月25日 · @Scratch---Cat, what are you trying to express when you say "have a good day"?I'm most accustomed to hearing this as a generic kind of "goodbye", usually in a retail setting, in which case the Japanese would be very context dependent -- often something like ありがとうございました (arigatō gozaimashita, "thank you [for coming]"), or またお[越]{こ}しください (mata o-koshi ...

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2020年7月28日 · yoi yume o / ii yume o. is a phrase that can be taken to correspond to "sweet dreams" which can be used in combination with おやすみ(なさい). (良い can be read either よい yoi or いい ii.)

2014年7月13日 · 「よいではないか」 literally is "Isn't it OK", and usually it means "That's OK" or "No problem". In general, 「よいではないか」 is a pompous expression and used by high-ranked people, such as kings, mainly in fiction.

The original form is definitely よい, and that's what you'll find in old texts. As often happens with common words, the pronunciation was simplified a little in its most common form, the Rentaikei form (which is the dictionary form), and became ええ in western dialects (Kansai-ben) and いい in the Tokyo dialect, which serves the basis for Standard Japanese.

い/よいだろう < i/yoi de ar-ou (possibility form ou attached to the verb ar) 'it is probably good' い/よいでしょう < i/yoi des-you (possibility form (y)ou attached to the verb des) 'it is probably good' (polite) よかろう is slightly old fashioned, but is still in use …

You wouldn't say yoi in a casual conversation. If you use he it should be in a context that allows it, which definitely isn't "sweet dreams". よい旅を。よい明日へ。御運を。 Oh, omitting the verb is more frequent in written expressions, in spoken language their usage is very limited. –