Fasting is generally done for religious or spiritual purposes. There's really no benefits to it from a weight loss/fitness perspective.
Here's the expert answer for fasting:
Will fasting jumpstart my weight loss efforts and boost my health? www.sparkpeople.com/community/ask_the_expe
"As a result, you might lose weight, but it is due to water loss, dehydration, and muscle tissue wasting, and is usually accompanied with symptoms such as fatigue and dizziness. "
There's no evidence that it helps, and a great deal of evidence that it can hurt. I have no problem with a fast for religious purposes (and have done them myself, but did not track weight and it wasn't about health at the time), but it's not a wise tool to try and use to "get healthy", because by definition it has the opposite effect.