I wouldn't recommend a juice fast, due to all the sugar you'll be ingesting. And I'm not sure about doing a fast for that long. Some people have gone on fasts here for a week and no longer, and reported some astonishing healings: ridding themselves of asthma and eczema to start! Most people fast to burn excess fat we're having trouble with losing, but also for health benefits. I'm currently fasting due to digestion issues I've been having.
Drink only water on a fast.