The day of your wedding is usually a blur of activity starting early in the morning and going until late at night. It is very easy to get so caught up in everything that you can easily forget to do one simple thing - eat. Don't skip this, no matter what. Eat breakfast - even if you have to be up ungodly early to start your hair and makeup, have something; a bagel, a donut, something healthy if you can manage.
If there is time before the ceremony, plan on having a plate of sandwiches or something simple that you and your bridesmaids can eat before they get dressed that won't mess up their makeup. If you have a caterer for the reception, ask if they have a simple lunch tray they can add on for you to have earlier.
At the reception, actually sit down and eat. Yes, you want to run around and see your guests, yes, your great aunt will insist on hanging on you and telling you her life story, but try appointing a bridesmaid or family member to be your "meal guard" and have them intercept anyone who wants to bug you while you are eating. You took lot of time to plan your menu - you should enjoy it.
After the evening is over, even if you did get to sit and eat, your nerves and adrenaline will still be running, so you will probably be hungry by the end. Having late-night snacks or meals available for the after-party is becoming a new trend that helps fill in any food gaps (and helps with the guests who may have had a bit too much to drink). Once again, ask your caterer if they have any suggestions, or your venue may have advice on what kind of late-night food is available.
Bottom line: If you don't keep up your food intake throughout the day, your energy will drop, you can get 'hangry', and even run the risk of getting lightheaded or dizzy during something important - like saying your vows. Don't think you can rely on a big caffeine buzz to keep you going all day (although the Starbucks cup with "Bride" written on it is pretty awesome). If you can't commit to meals, keep snacks on hand to keep eating little bits throughout the day to keep your blood sugar level and let you enjoy the day without worrying about feeling strange. - And don't think that starving yourself will make you look better in your dress. If your seamstress has done her job properly, you will look amazing even if you eat something.