Vocabulary: the difference between ‘by’ and ‘until’
Intermediate level (B2-B2)

Looking at some examples should help you understand the difference:

by

This is often used when there is some kind of deadline or time limit eg when you need to finish a piece of work for college or work.

I have to finish the essay by Friday.
The report needs to be completed by June.
The party will have to finish by 11pm because my parents are returning home at midnight!

In these examples, there will be a problem if the essay or report is late. However. it is very possible that the essay or report will be finished early eg on Wednesday or at the start of May.

until

This is used for something or situation that will continue until a fixed time or date:

He’s president until 2017.
I’ll be living with my parents until July. After that I’m going abroad to work.
Don’t worry. Until I hear from you, I won’t make a final decision

However, ‘until’ can sometimes be used like ‘by’. For example:

You have until 8.00 to tell me the truth (spy movie).
I have until tomorrow to make a decision about accepting or turning down the job.