Fasting and an Ayurvedic view

Fasting
Fasting is abstinence or reduction from some or all food, or drink or both, for a period of time. An absolute fast or dry fasting is normally defined as abstinence from all food and liquid for a defined period. Fasting was generally considered a spiritual practice, not intended to cleanse or detoxifying the body. Modern concept of fasting is generally focused on lose weight (by reducing calories intake and nutrition) or determined by health conditions, sometimes is forced by situations (lack of food) or prescribed. 
Before modern times, our access to food could change on a day-to-day basis depending on weather, seasons, wildfires and other factors. In those days (sometime this days too) intermittent periods of fasting were a natural occurrence. 
Daily, we are fasting at night when we sleep (that is if we have fully digested before going to sleep), this gives the body a chance to fully concentrate on repairing, rebuilding and eliminating dead cells and toxins (hence bowel movements should happen in the morning). 
In modern life, we don’t have a natural period of fasting:
We eat when we're stressed
We eat on the run
We overeat
We skip meals
We eat at irregular times
We eat bad food combinations
We eat heavy foods that are difficult to digest
We eat our next meal before our previous meal has digested
We eat too much, too late at night
We are still digesting when we go to sleep
{clue: how important is routine}

As a result, it is inevitable that our digestive fire {Agni} will become imbalanced and toxins {Ama} accumulate. Imbalanced Agni and accumulation of Ama are considered the root cause of ALL disease in Ayurveda.

What is the Ayurvedic view on fasting?

Fasting is used in Ayurvedic tradition as a means to rest the digestive tract and give the body a chance to release old stored toxins.

Fasting or cleansing diet, in Ayurveda, is the use of food with high nutritional value to support the self-healing and natural detoxification of the body. 
In Ayurveda each individual is different, hence one diet, one recipe, one detox/fast doesn’t work for all, in some might help the same way that to others might not.

Lesson 5 on Sister science explain how different Dosha suit a different cleansing diet so that their imbalances are not aggravated.

If you have been on a diet before, how this made you feel, did it satisfy your appetite or were hungry all the time? Did it make you constipated or the opposite? Did you feel tired or bright? (food has an effect on our mind too).
Some diets create a yo-yo effect, effective in loose weight but then create a different imbalance that the body reacts and start putting on weight again.
If you have tried different diets, the fact that you have, automatically tells me that it didn’t work, or you wouldn’t need to try a different one. 
As you can see this subject it's quite complex and before you do any fasting/detoxification, understand your imbalances first, your Dosha.

Complete fasting is not recommended and can send the body into a state of crisis, should be supervised by a health professional.

Regarding cleansing or detox diet, much depends on your Dosha: for example Vata people should avoid a long period of cleansing diets; juicing or raw diet in Autumn is a big no-no for vata. 
To find a cleansing diet that suits your Dosha, its best to Consult an Ayurvedic professional. 
For a home cleansing, generally a kitchari mono diet is recommended during the change of seasons but again the length of the cleansing and recipe vary slightly depending on your Dosha. A daily cleansing routine should support the cleansing diet too to be more effective.