I think there's something about a fixed deadline that motivates -- it has to get done, and it has to get done by this specific date. Without a deadline, its easy to dawdle, let yourself get distracted, and let other things take priorty, because after all you can finish it up any time, right?

I know I've done more tatting since Thanksgiving than I have for nearly a year, because I set myself the deadline of getting them done in time to go out with holiday cards. (Which has, sort of, happened.)