The first day of the fast is always the hardest. Day two is not as bad because your body is adjusting. Day three you began to get weak, but your appetite is slower. For me, it is easier to fast during the fall or winter months. And over the weekend or during vacation that way I have time to exercise and rest. Honestly you may not feel like exercising depending upon the type fast you practice. You can fast longer days if you drink mineral water or drink raw juices. Lastly, when you come off a fast never eat heavy foods immediately. Rather, the day you break the fast eat soup and drink lots of water.
Nykkii provided some good info. You should also research the various types of fast. I too suggest caution and moderation.
I have been fasting sparingly for years. Although I do not fast to lose weight when I first started fasting I got skinner. Yep, after a good internal cleansing, I was looking anorexic. At that time however, it was easier to lose than to gain weigh. Some models fast to lose a few pounds for a photo shoot, as it is a quick way to shed pounds if you are already slim and not overweigh. But fasting to lose weigh is not healthy in the long run. Nor will everybody have the same amount of weigh lost. It depends upon the structure of you body and metabolism. Fasting is more effective for cleansing.