I mailed the letter today, so they should receive it by Monday.
(= on or before Monday, on Monday at the latest)
We’d better hurry. We have to be home by 5 o’clock.
(= at or before 5 o’clock, at 5 o’clock at the latest)
We cannot use “until” with this meaning.
Tell me by Friday whether or not you can come to the party.
We use “until” or “till” to say how long a situation continues.
Shall we go now? No, let’s wait until or (till) it stops raining.
I was tired this morning, so I stayed in bed until half past ten.
Compare “until” and “by” in these sentences:
Sue will be away until Monday,. (so she’ll come back on Monday)
Sue will be back by Monday. (she’ll be back on or before Monday, on Monday at the latest)
You can also say by the time (something happens)
It’s not worth going shopping now. By the time we get to the stores, they will be closed.
(= they will close between now and the time we get there)
When you are talking about the past, you can use by the time (something happened)
Tom’s car broke down on the way to the party last night. By the time he arrived, most of the guests had left.
(= it took him a long time to get to the party and most of the guests left during this time)
It took them a long time to find a place to park their car. By the time they got to the theater the play had already started.