People don't recover well or the surgery won't go well.
There has been research into the full moon effect on an assortment of human events including surgery. We know that the moon affects tides and women's menstrual cycles. Many policemen swear that a full moon means an increase in crime. Some doctors do not schedule surgeries during the full phase of the moon but they rarely want to go on the record as to why. Go to your local Medical School library and look it up. There have been many articles over the years in such publications as The Journal of American Medicine, The New England Journal of Medicine etc.