Here's a GREAT article on eating before and the day of a tri, same principles that apply to your compitition.
Tri nutrition. (wfs)
I'll add that last weekend I had a half marathon and I followed this whole "72 hour prior eat a ton of carbs" thing, and I had SO MUCH energy pumping through me by Saturday night I didn't know how I would sleep that night, and Sunday (the morning of the marathon) I had a TON of energy and was ready to go.