The leader running the challenge will post when the closing date and time is and normally you get a couple of posts notifying people of a day to go. Often it is late because we have life / work commitments but once its closed then the challenge threads get locked and the voting poll is put up. So you can start a map on the last day and work on it right up to the wire so long as you get your final updates in before the thread gets locked. Often tho its better to work on it a bit through the month or else people wont follow it and appreciate the map. But if you were to put in a super map on the last day then its possible it could win. You would need to show that you did the map yourself with some progress images as well.