Let me know: He's on vacation from Monday until Friday.

[Sosashi]

hi

1. he's on vacation from Monday until Friday.
when does he have to go to his company?

2. he's on vacation until Friday?
when does he have to go to his company?

I'm confusing SO MUCH.
please let me know.

according to dictionaries, "until Friday" stops continuing on Friday.
the vacaion stops or change at the end of Friday(=Saturday 12 a.m.)
I think he must go to his company as of saturday.

thank you for your consideration in advance.

[Milanya]

untill Friday - he is back at work on Friday morning
through Friday - he is back at his desk on Monday morning (Saturday and Sunday are days off and midnight is not usually a sutable time for work).
If you really want to be clear, say: "He is on vacation. He will be back on Friday (or on Monday)."
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con·text - The part of a text or statement that surrounds a particular word or passage and determines its meaning.

[NikJames]

he is on vacation until friday can mean that he is returning anytime on friday... It could be early in the morning, it could be late at night (it can also mean that he will be gone from work until friday, implying that he will be home thursday night). It is not clear by the sentence.

If you want to be clear I recommend what Milanya said and say exactly when he will be back.

Cool American English

[Sosashi]

I've read your reply to my questions. thank you so much.
I'd like to ask you questions again.

"he worked for our company until 1999"

does it mean that he stopped working in 1998? otherwise in 1999?

"he waited for me until 8 a.m."
does it mean he stopped waiting at 7:59 a.m. before 8 a.m.? otherwise at 8 a.m.

[Milanya]

Sosashi, you are splitting hairs.

"He worked for our company until 1999" - He quit somewhen during 1999.
"He worked for our company through 1999" - He quit at the very end of 1999. He did not come to work on January 1, 2000.
"He waited for me until 8 a.m." - He stopped waiting at 8 a.m. (with the 8th stroke of the clock).
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con·text - The part of a text or statement that surrounds a particular word or passage and determines its meaning.

[Azhdar]

in my Idea he is back at work on Friday morning