Fasting isn't mean to be a quick fix solution. If your only goal is to look weight for Halloween then what will happen is that after you Halloween. You'll get fatter then you were before you fasted.
Think clearly before you head down this path. If you don't change the way you eat or look at food then get prepare for a life of being fat. Fasting is a great tool for losing weight and starting a new life but it's a double edged sword.