by vs until

Both until and by indicate “any time before, but not later than.”

Until tells us how long a situation continues. If something happens until a particular time, you stop doing it at that time.

For example: 

They lived in a small house until September 2003.
(They stopped living there in September.)

I will be away until Wednesday.
(I will be back on Wednesday.)

We also use until in negative sentences.

For example:

Details will not be available until January.
(January is the earliest you can expect to receive the details.)

If something happens by a particular time, it happens at or before that time. It is often used to indicate a deadline.

For example:

You have to finish by August 31. (August 31 is the last day you can finish; you may finish before this date.)

We also use by when asking questions.

For example:

Will the details be available by December?
(This asks if they will be ready no later than December.)