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.)