"By and Until"

There is often confusion between the use of by and until. A common mistake is:

"Please, can you send it to me until Friday?" (it should be by Friday)

So when do we use by?

We use by to set a time limit/deadline for completion:

"It must be ready by Friday."

It can be ready before Friday, but Friday is the final day.
(By is a preposition here.)

and when do we use until?

We use until when the activity continues throughout the period up to the time limit/deadline:

"I'll be here until six o'clock."

Between now and six o'clock, I shall be here all the time.
(Until can be a preposition (until five o'clock) or a conjunction (until you have finished.) )