Drinking half a gallon to a gallon of water is cool but don't do it for too many days at a time, you can actually get water poisoning. On career day in Medical Studies someone actually asked the question if you can drink too much water, and they said you can. I can't remember systems, but it's possible to get a bladder infection or something or other because you're pissing TOO much.
If you're going to binge on the water, drink half a gallon to a gallon a day for about 3-5 days, then stop.

Another thing is, a friend of mine drank an entire gallon of water one day before his drug test, and he had drank so much water that he actually had more water than piss in his system or something. They threw out his sample and asked him to come back the following week.

So, I'd stop the day before the test, if not 2 days before.