While both 'by' and 'until' are used to indicate actions in time, there are essential difference between the two usages.
'until': an action or event which will occur any time before the time period that usually follows the word, but not beyond that time. The word 'until' is used to indicate the time period that a situation or event has been going on for. E.g.: I worked until 3 a.m. last night. It is also used to denote negativity. E.g.: There will be no voting until the day after tomorrow. Rate is valid until 31st March 2009 or Rate is valid up to 31st March 2009
'by': an event or a situation that happens at or before a given time. For example - You have to close your shop by 9 p.m.