Until vs. By
If something happens "until" a particular time, you stop doing it at that time.
例：We lived in a small apartment until this September.（We stopped living there in September.）
If something happens "by" a particular time, it happens "at" or "before" that time. It is often used to mean a deadline.
例：You must turn in your homework by August 31.（August 31 is the last day you must turn in your homework; you can also turn in your homework before August 31.）