Abstract Background: Previous studies showing higher mortality after elective surgery performed on a Friday were based on administrative data, known for insufficient case-mix adjustment. The goal of this study was to investigate the risk of adverse events for patients with colon and rectal cancer by day of elective surgery using clinical data from the Dutch ColoRectal Audit. Patients and Methods : Prospectively collected data from the 2012–2015 Dutch ColoRectal Audit (n=36,616) were used to examine differences in mortality, severe complications, and failure to rescue by day of elective surgery (Monday through Friday). Monday was used as a reference, analyses were stratified for colon and rectal cancer, and case-mix adjustments were made for previously identified variables. Results: For both colon and rectal cancer, crude mortality, severe complications, and failure-to-rescue rates varied by day of elective surgery. After case-mix adjustment, lower severe complication risk was found for rectal cancer surgery performed on a Friday (odds ratio, 0.84; 95% CI, 0.72–0.97) versus Monday. No significant differences were found for colon cancer surgery performed on different weekdays. Conclusions: No weekday effect was found for elective colon and rectal cancer surgery in the Netherlands. Lower severe complication risk for elective rectal cancer surgery performed on a Friday may be caused by patient selection.