It's so important to practice your race nutrition! Never a good idea to try something new on race day. Long run and/or long ride days are good opportunities to practice pre-race dinner and breakfast... plus race nutrition.
Everyone's different and can handle different things. For me what's always worked well for a pre-race dinner has been fish, cooked veggies, and a fist size serving of pasta. I try to eat before 8pm the night before a race. (If it's going to be a hot race I salt my food and drink extra water race week.) For breakfast (longer races 1/2 marathon, 1/2 iron, full iron) I eat a whole wheat bagel with peanut butter and a banana plus a big glass of water... Ironman I added a glass of juice. For a sprint or olympic race usually just the bagel and pb. I eat breakfast 2-3 hours before my wave starts. Depending on the distance I may take a gel about 10 minutes before my wave starts. I've worked with dietitians while training for my Ironman races and it really helped me dial in my pre-race and race day nutrition.
Race day nutrition greatly varies depending on the distance. A sprint you may only need hydration or a small amount of calories for a boost at the end. Olympic distance - typically once maybe twice on the bike (nutrition) and once more halfway through the run. Half iron and full iron you'll obviously need more nutrition... generally the rule is 100-200 calories an hour. You have to experiment with how much your body can handle and what types of products work well for you. For the full iron on the bike I typically take in small bites of Clif bar plus Accelerade or Tailwind. Run I only do liquid nutrition. The longer you are going the harder it is for your stomach to digest since your blood and energy is working so hard to keep your brain and muscles functioning. Keeping a close eye on your nutrition timing is pretty important during a race.
Again, everyone is different and what worked for me may not work for you... That's the beauty of training... you have the time to try out what will work race day.
Good luck to everyone this season!!