Are you limited by space for cooking? If not, I would wait and put the brisket at about 8:00 or so tomorrow. An 18 lb brisket is HUGE and I would plan on 1/5 hours or more per lb to cook. With that said, if it cooks quicker, double wrap in foil, wrap with a towel, and put in an ice chest. It will rest comfortably for 4-5 hours and still be ready for slicing/eating. If you feel compelled to start tonight and it finishes way early, then reheat in an oven at about 225 or so and make sure it stays in the foil, that way it will be nice and moist. Just so you know, we want pictorial evidence of how this feast turns out!!!
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