We should start this one off by stating what might be obvious to most. Race day nutrition is crucial.
Be fore we get into the answer, It's important to point out that your diet should not all of the sudden change the day of, or even the day right before a race. Daily nutrition is something an athlete should be aware of as part of their lifestyle.
With that said, your pre-race intake of fluids and calories will make a difference. I'll start with a couple common mistakes that you may regret during the race.
The first is, you really don't want to overdo it with the water. Trying to drink as much water as you possibly can is a huge mistake. Yes you need to be properly hydrated, but you're not a camel who save the water for later when you need it. Proper hydration is what you should be doing on a daily basis, so all of the sudden drinking a ton of water will basically dilute the electrolytes in your system, causing cramping or even worse.
The second is, loading up on pasta. For some reason people think they need to eat more pasta than what you would find in the kitchen of some Italian restaurant. It's a bad idea. You should only take in an amount of carbs that your body is able to process. Otherwise, you're simply running on a full stomach.
Now as for what to eat on race day, The exact ingredients that go into your meal will depend on what you like. However there are some foods that are recommended. If breakfast is going to be the big meal before the race, avoid that big plate of bacon, eggs and hash browns. Perhaps go with something like some cereal that doesn't contain a lot of sugar, wheat toast (some butter is okay), fresh fruit, and of course water.
If it is going to be lunch, try some kind of lean deli sandwich, preferably turkey. Try to choose whole grain bread to go with that. You could also fruit or applesauce (the natural stuff). A little bit of milk is okay too.
It's a menacing topic for some. Do I eat more? Do I eat less? What will give me enough energy? What will sit like a rock in my stomach?
You'll definitely want to get some carbs. Actually carbohydrates should make up roughly 80% of the calories you consume in that meal. In fact, it's a good idea to lessen the fat, protein and fiber intake, allowing more space for carbs. You should also avoid eating foods that produce gas, like some kinds of beans, onions, etc.
When you eat matters as much as what you eat. There are different studies, and everyone seems to have their own way that works for them, but it's smart to avoid timing that meal inside two hours before the race. Really, four hours before is more ideal, as it gives your body enough time to digest properly, as well as process the calories and store the energy. Four hour ahead seems like a long time, but in most cases it isn't too long, where you would need to worry about burning that stored energy. If your race starts early in the morning, and you want to get enough sleep, just try your best to eat 2-3 hours beforehand.