I recommend counting calories—not forever, but for about two weeks—so you get a feel for what a regular day of eating feels like. I have used and like Cron-o-meter, Lose It!, and Fitbit to track calories. Don’t get bogged down in the details. If a type of food you ate is not in the database, find something similar or make an educated guess. You just want to make sure you’re in the ballpark.
Aside from these official days of fasting, Jews may take upon themselves personal or communal fasts, often to seek repentance in the face of tragedy or some impending calamity. For example, a fast is sometimes observed if a sefer torah is dropped. The length of the fast varies, and some Jews will reduce the length of the fast through tzedakah, or charitable acts. Mondays and Thursdays are considered especially auspicious days for fasting. Traditionally, one also fasted upon awakening from an unexpected bad dream although this tradition is rarely kept nowadays.
It’s a good idea to weigh yourself before starting (and then again at the same time each week), and also to take chest, waist and hip measurements. The rate that you’ll lose depends on your age, levels of activity and your own body – so don’t be disheartened if you’re losing a pound a week while others are losing five or more. Typically weight loss will be faster at the beginning of the diet, and for those with a lot of weight to lose to reach a healthy weight (see this calculator to find out what a healthy weight - or BMI - is for someone of your height and weight).
Fasting also helps us learn the lessons of the Day of Atonement : forgiveness, reconciliation to God and the need to resist Satan and pray for the time of his removal (Revelation 20:1-3 Revelation 20:1-3 1 And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand. 2 And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years, 3 And cast him into the bottomless pit, and shut him up, and set a seal on him, that he should deceive the nations no more, till the thousand years should be fulfilled: and after that he must be loosed a little season.
Well, most notably, it’s a great way to get lean without going on a crazy diet or cutting your calories down to nothing. In fact, most of the time you'll try to keep your calories the same when you start intermittent fasting. (Most people eat bigger meals during a shorter time frame.) Additionally, intermittent fasting is a good way to keep muscle mass on while getting lean.
Fasting is often used as a tool to make a political statement, to protest, or to bring awareness to a cause. A hunger strike is a method of non-violent resistance in which participants fast as an act of political protest, or to provoke feelings of guilt, or to achieve a goal such as a policy change. A spiritual fast incorporates personal spiritual beliefs with the desire to express personal principles, sometimes in the context of a social injustice.