Once or twice per week you can have a treat. Remember you will have 600-1000 calories to work with in the main meal. This should make it pretty easy to enjoy some of your favorite foods or go out to a restaurant with friends/family. Just make sure to not go over the calorie budget for the day. If you do end up going over the calorie budget for the day then you can compensate by slightly reducing calories the following days.
The good news is there are things you can do to help alleviate those symptoms. Firstly, you should see your doctor before going on the diet to get the thumbs up. Then, if you're experiencing such symptoms and they're not the result of something else (like the actual flu), start by drinking plenty of fluid, while avoiding activities that can dehydrate you such as an intense workout. Dr Evelyn Lewin also suggest increasing your salt intake by adding half a teaspoon to two teaspoons of salt into your food. if you're able to persist through these symptoms, you’ll quickly transition through this stage and come out the other side bursting with energy.
The first modern study of fasting as a treatment for epilepsy was in France in 1911. Twenty epilepsy patients of all ages were "detoxified" by consuming a low-calorie vegetarian diet, combined with periods of fasting and purging. Two benefited enormously, but most failed to maintain compliance with the imposed restrictions. The diet improved the patients' mental capabilities, in contrast to their medication, potassium bromide, which dulled the mind.