Ketosis is when your body does not have any more food to digest so it starts feeding on stored fats. It will also feed on dead cells and clean out your body so to speak. This is what is meant by cleansing the toxins.
When you eat anything that requires digestion you basically start all over again. It takes anywhere from 12 to 48 hours to go into ketosis depending on how much you have in your system.
With that said, mistakes happen. Depending on what your goal of the fast is you may want to go a short time or you may want to go for a longer fast. I myself did many smaller fasts before I started this 30 day fast. I am on day 24 of a 30 day Water Fast so I don't do the broths or anything, but if that helps you get through rough patches I say go for it.
I personally skipped my communion at church last weekend because we do it once a month and I knew that it would start my digestion all over again. I said the prayers and I am sure God understands why I did not partake, but I did keep in mind the sacrifice and the reason for communion.
Good luck. I hope this helps you. Also search on the internet for fasting information. It is plentiful and very informative.