Background: Although the “weekday effect” may influence postoperative outcomes, controversies regarding its existence remain. The aim of this study was to investigate the association between the day of the week the surgery was performed and postoperative outcomes. Methods: Data from 58,646 patients who underwent elective abdominal surgery at Seoul National University Bundang Hospital from May 2003 to February 2018 were retrospectively analyzed. Two elective surgery groups comprising of 33,513 (57.1%) and 25,113 (42.9%) patients who underwent surgery in the early and late days of the week, respectively, were analyzed. Late days of the week was defined as days where surgery was performed within 2 days before weekends or holidays and early days of the week as all other weekdays. These groups were balanced using inverse probability of treatment weighting. The adjusted 30-day mortality and length of hospital stay were compared. Results: In the weighted sample, all 52 clinical covariates were comparable between the 2 groups. The weighted sample analysis did not reveal a significant difference in the 30-day mortality between early days of the week and late days of the week (0.19% early days of week vs 0.18% late days of the week; relative risk, 0.97; 95% confidence interval, 0.66–1.50; P = .82). Similarly, the median length of hospital stay showed no significant difference between the 2 groups (median value for both groups, 5.0 days; interquartile range 3.0–8.0; relative risk, 1.00; 95% confidence interval, 0.97–1.03; P = .16). Conclusion: A weekday effect associated with the day of elective surgery performed was not found in this study for either the 30-day postoperative mortality or length of hospital stay.