"I can't leave on Tuesday. I won't be ready until then/by then". Which ending is correct? Why?

Question

Answer

If you can't leave on Tuesday, then the correct ending is 'by then', because 'by then' means that on Tuesday you will not be ready to leave, while 'until then' implies that you will be ready to leave on Tuesday.

By then = before a specific time or date.
Until then = not before a specific time or date

Answer

look at it: http://www.italki.com/question/179952