Here’s a thread that might help get you started: https://www.reddit.com/r/Dryfasting/comments/yyjab6/4_day_dry_fast_before_surgery/?utm_source=share&utm_medium=android_app&utm_name=androidcss&utm_term=1&utm_content=share_button
Unless a person has dry fasted previously, I would never recommend doing it right before a major surgery. I might suggest water fasting if the person was insistent on doing any fasting at all for surgery prep. Most surgery preps are going to put you on IV and likely obliterate any metabolic fasting effects you might have desired as you go into surgery.
Water fasting also makes sense as a recovery protocol after surgery, when your body struggles processing the heavy anesthesia loading you’ll have undergone, unless you’ll only have been on a local for outpatient surgery. Still, the body will be dealing with a lot of toxins at that time. Best to be gentle.
Overall, unless you’re a well-practiced veteran of fasting with a body fully adapted to it, I wouldn’t try and jump start your experience of fasting before a major surgery, but I certainly would use fasting in some measured way after the surgery to speed my recovery.
Hope this helps your thinking.