What will happen if a person eats only once a day?Contrary to popular thought, we can digest everything in huge meals given enough time. So one large meal that covers your calorific (macronutrient) needs and mineral/vitamin (micronutrient) needs is fine for maintenance - if you are an otherwise healthy individual. If you want to lose bodyfat, one meal per day could be a good strategy as you could limit your food intake all day long, then have a blow out meal before retiring for the night, but still remain a few hundred calories lower than your maintenance calories (about 2000 calories) for an average man. By the way the "calories" spoken of are usually referring to kilo calories - kcals. Look it up if you need a better definition.
One problem with eating once per day to maintain your weight might be the amount of food you would have to eat at one sitting. Adding extra healthy fats could be the solution as fat provides a lot of calories (9kcal per gram compared to carbs and protein at 4 kcals) and takes up a lot less space in your stomach. If you tried to get 2000 kcals from leafy vegetables in one sitting you might pop! You would have to eat a bucket load - literally - as they are almost all water and fibre.
At my physiotherapy practice in Northampton - Brackmills Physio Clinic, we work with some athletes who stick mainly to one large meal in the evening, because of the nature of their training. Some MMA fighters we work with, avoid food during the day as they need to feel "light" when training. Hydration is very important though, so going without water is counterproductive.
Psychologically, it becomes quite easy to go without food during the day, when you look forward to a "reward" in the form of a large meal at night.