For 20 days I am sure you will be fine as long as you don't have any exams or anything of the sort that requires concentration. Unless you have a medical condition, then I would suggest consulting a doctor. Also, it's not good to continue if you start feeling bad.
I have done very strict detox diets, but not fasting. A part of one of the diets was not to eat anything for 24 hours once every week to give your digestive system a break. I did not mind. Drank tea a lot.
I really want to go on a spiritual cleansing fast, but that's after I lose some weight. In my experience short term restricted dieting actually made me gain weight significantly when I came off of it.
Well, good luck
