When you use "by" in sentences like "He won't be back by Friday." or "They won't publish my book by March" you are saying the desired event could occur any time after the specified time. In these same examples, if you replace "by" with "until" you are saying that he will be back no earlier than Friday, or the book will be published no earlier than March. In using "until" you are being specific on exactly when the even will occur. Tell me if this makes sense or not.
