Grammar explanation for ~よし

Question

Studying for the N1 with sample questions I stumbled across this grammar construct ~よし.　However I am unable to find any explanation online or in any of my grammar books - probably I am searching for the wrong expression, but maybe anyone here can help.

Here are a few sample questions, that I could not answer so far:

「この夏、定年退職なさった（と・の・との）よし。長い間、お疲れ様でございました。」 This summer, I will reach the age for retirement (?). Thanks for taking care of me for so long.

「来月、日本にいらっしゃる（よし・べし）。ぜひ、お会いしたいですね。」 Next month I will be in Japan (?). I definitely want to meet you.

Answer 1

First of all, this 「よし」 is 「由」 in kanji. (It has nothing to do with 「良{よ}し」 meaning "good".)

All by itself, 「由」 means "reason", "circumstances", etc. Notice the kanji 「由」 is surely used in the word 「理由{りゆう}」 ("reason").

「由{よし}」 is most often used in formal or business letters in the form of:

「Mini-Sentence + （との） + 由」

meaning:

"I hear that [Mini-Sentence], (so I am replying.)"

This pattern is rarely, if ever, used in informal letters/emails between two young friends.

「この夏、定年退職なさった（と・の・との）よし。長い間、お疲れ様でございました。」 This summer, I will reach the age for retirement (?). Thanks for taking care of me for so long.

It is not the writer who has retired. It is the receiver of this letter. 「なさった」 is honorific.

"I hear that you have retired this summer."

「来月、日本にいらっしゃる（よし・べし）。ぜひ、お会いしたいですね。」 Next month I will be in Japan (?). I definitely want to meet you.

It is not the writer who is coming to Japan. The writer is already in Japan.

You would not use the honorific 「いらっしゃる」 to talk about your own action.