What should I eat on race day?

Long story short: everything, just in the right amount

CARBOHYDRATES

Most endurance athletes and coaches know how important it is to maintain the body's carbohydrate (glycogen) stores. They are also aware that, despite the fact that the depletion of glycogen may be delayed to a degree, by consuming carbs during activity, the body's stock capacity can only provide enough energy for about 90 to 180 minutes before fatigue sets in. 
It is when these stores are exhausted that runners 'hit the wall', cyclists 'bonk' and everybody is doomed to slow down. Some even to quit.

It comes without saying that prolonged exercise forces athletes to get the energy they need from additional sources. The basic rule is to load low glycemic index carbs 3 or 4 days before the race (will digest slowly) and high glycemic carbs during the race so that the body can absorb fast and efficiently.
Here is a table that you better remember by heart.

When?	How much?	Why?
3-4 hours before	2-4 g/kg bodyweight  = ± 210g = 840 Kcal	·   Liver glycogen will be filled ·   Blood sugar will be maintained ·   No hunger feeling
1 hour before	1-2 g/kg bodyweight = ± 140g = 560 Kcal	·   Keep the glycogen reserves high
During	0,7 g/kg bodyweight every hour of sport in intervals of 15-20 min = ± 49g =196Kcal/ hour è 50 kcal every 15 min NB: You can go until 70-75 g/hour = 280- 300 kcal / hour è 70- 75 kcal every 15 min	·   Keep blood sugar high ·   Increase carbohydrate oxidation ·   Delaying fatigue
Recovery (until 2 hours after race)	1,1-1,2g/kg bodyweight (preference for high GI) = ± 84 g = 336 kcal	·   Recover the blood sugar value ·   Resynthesis of muscle glycogen ·   Restore muscle tissue

Main sources:  Whole grain, apples, bananas, peaches, asparagus, carrots, rice milk, lentils, just to name a few.

PROTEINS

Proteins alone do not provide energy unless glycogen and fat are depleted and like fat, they can slow down your metabolism. You will typically get enough protein for race day if you eat high quality proteins, such as whey, egg whites, milk, soy, as part of your daily intake. Small amounts of protein during a race may reduce muscle damage. Recovery, however, requires protein. The recommended ratio is of 3 carbs to 1 protein.

Main sources:  Eggs, meat, fish, chicken, cheese, beans... But you probably don't want to go for meat ;)

FATS

Another good source of energy during exercise is fat. Fat has several advantages over carbohydrates. As a matter of fact, it yields over twice as much energy as carbohydrate (9 kcal v 4 kcal). Add the fact that our body is usually loaded with a good supply of fat and you get a consistent amount of calories and energy. Typically, the body has approximately 50-60.000 kcal of energy stored as fat compared to about 1500 kcal of glycogen. 
One of the effects of endurance training is to make the body a more efficient fat burning machine. Fat is not the enemy, if your body knows how to deal with it.
However, despite the abundant store of this substrate, it is not totally clear where the fat used in exercise comes from. Most fat in the body is stored in the form of triglycerides in adipose tissue. Some fat, known as intramuscular triglyceride, is stored directly within the muscle fibres. It is now believed that the greater utilisation of fat by trained endurance athletes comes from this source. This has a very important implication, because the amount of energy which can be derived from intramuscular triglyceride is thought to amount to only 2000-3000 kcal.

Depletion of the intramuscular triglyceride stores is now thought to limit endurance performance in much the same way as glycogen depletion. It has been shown that the replenishment of glycogen stores alone is not enough to restore maximum work capacity. Therefore, it may be just as important to restore, or strive to spare, the body's intramuscular triglyceride. 

Although fats are vital for physical activity, most individuals have enough body fat stored for energy during prolonged workouts. 
However, increasing the fat intake will only lead to less consumption of carbohydrates impeding glycogen stores (glycogen being the immediate source of fuel for physical activity). Moreover, fats have the tendency to slow down metabolism in general. Consider fats as a reserve tank of fuel, but keep them to a minimum during training and racing (20-30%).

Main sources:  Fatty fish such as salmon, nuts, cheese, avocado, …

Last but not least (and not to forget)

1.    Fibers: prevent gastrointestinal distress, they are difficult to digest, especially during high intensity training. Avoid whole grains and fruits with skin. In general, keep fibers to a minimum, at least on race day.

2.    Maintain proper fluid balance. This is vital for an optimal performance

Have a nice race!