An argument that surgeries seem to be significantly more dangerous on the weekends. One potential cause being more skilled surgeons not working then. There's plenty of evidence arguing no effect, but some counter arguments are that these tend to be with smaller sample sizes and have various flaws. One example from the article concerning mortality rates:
"Of these, 20/22 (91%) show the weekend effect, and 2 no effect (both neonatal mortality). The median relative risk (RR) of these papers is 1.11 (range 0.94-3.18), i.e. patients admitted at weekends are, on average, 11% more likely to die than those admitted in the week. This data takes into account confounding variables, as all these papers use regression analysis. "
I think it's pretty reasonable to try to schedule for Monday surgeries instead.