BY and UNTIL

BY and UNTIL

By (+ a time) = not later than
example:

• Tom posted the later today, so they should receive it by Monday. (= on or before Monday, on Monday at the latest)

You can not use until with this meaning:

• Tell me by Thursday whether or not you can come to the meeting. ( not 'Tell me until Thursday')

We use until (or till) to say how long a situation continues:
example:

• "Shall we go now?" "No, let's wait until (or till) it stops raining."