Quite bluntly, you're more likely to die.
These authors analyzed the 2008 Nationwide Emergency Department Sample, using 4,225,973 patient encounters as the basis of their observational analysis. The absolute mortality differences between weekday emergency department presentations and weekend emergency department presentations is tiny – about 0.2% difference. However, this difference is very consistent across type of insurance, teaching hospital status, and hospital funding source.
The NEDS sample did not offer these authors any specific explanation of the "weekend effect", but they expect it is due to decreased resource availability on weekends. The authors note specific systems in place (e.g., trauma centers, PICU, stroke centers) where weekend staffing is unchanged have demonstrated the ability to eliminate such weekend phenomena. However, it's probably never going to be the case that weekend shifts are less desirable – so we're probably stuck with this slight mortality bump on weekends.
"Don't get sick on the weekend: an evaluation of the weekend effect on mortality for