If somebody said to me, "I have to work until Friday", would that mean that he still had to work on Friday or would that mean that Friday was already his day off?
This is a really good question. Technically, you can't know for sure, because both situations (finished at the start of Friday, and finished at the end of Friday) could be described that way, and have been. In my (Western Europe) experience it's much more likely that Friday is included and not excluded. But if somebody said "we can party until Monday" you would certainly not expect a three-day weekend, right?
In practice, you can probably fill it in from context: i.e. you either know that your friend works Monday to Thursday only, OR you know that Mon-Fri is the normal Western work week.
Recently I've been wondering whether Friday night is a "weeknight". Go figure.
If you say "I have to work until 2:00:00 PM tomorrow.", you have identified an instant in time, and the statement is precise.
"Friday" describes a long span of time, so "I have to work until Friday." is not precise, and the meaning must be divined from context, or made more specific, as in these examples:
"I have to work until Friday at 5 PM. Then I will be free to party." or
"I have to work until Friday, so I can leave for Vegas on Saturday morning." or
I disagree with the two answers above.
"I have to work until Friday" unequivocally includes at least some part of Friday.
You might be free at (say) midday on Friday, but you're not free all day.
Ditto with "we'll party until Monday": the expression implies that our partying will last at least into the small hours of Monday morning.
Just as "Lily will practice guitar until her fingers bleed" involves the future loss of at least small amount of blood.
In relation to "out of office" e-mails, the message "I will be out of the office until Friday, 22 October" explicitly says the author won't be in the office until at least some portion of Friday, 22 October has elapsed. However, it effectively implies that they'll arrive back in the office at some time during the working day (if not stated otherwise, the reader will probably infer that they'll be back at the normal time on Friday morning).
Funny enough, we have the same problem in Italian, with the expression "fino a..." that's similar to "until..."/"up to..."
One solution adopted (in Italian) has been to add a final "included" or "excluded" to make clear if the date/time mentioned (more often a specific day) is included or not. The final result would be like (forgive my literal translation, please) "I'll be on vacation until Friday, included": does this form make sense or look awkward in English?