Summary. In closing, although we've established that “until” seems to include the date that follows it, you may want to avoid using “until” with a deadline or specific date if it's possible that someone will misunderstand you.

Does Until include the date stated?

Until, which means up to, generally includes the specified moment. The kidnappers have given us until October 11th to deliver the documents.

What does it mean until Friday?

"Until Friday" is a range of time. I have the option of acting any time in this range. It starts now and it ends on Friday. It means "They require my decision on or before Friday." "I have to let them know until Friday"