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