By and until

By is used for an action or event that takes place before a specified deadline. The event or action can take place before the deadline or on the deadline at the latest.

Until is used for an action or event that takes place over a specified period of time. It lasts for the whole of that time.

By

• Please send me the report by Friday.
• Can you call me by 25 August, so I know what’s going on.
• I’d like you to arrive by 10:00 am, so that we can start the meeting on time

Until (or till)

• The meeting starts at 10:00 am and lasts until lunchtime.
• His contract runs until the end of the year.
• Could you keep checking Sarah’s emails until she gets back from holiday.