Importance  The phenomenon of a weekend effect refers to a higher potential for adverse outcomes in patients receiving care over the weekend. Few prior studies have comprehensively investigated the effects of postoperative weekend care on surgical outcomes in a generalizable cohort.

Objective  To examine differences in short-term and long-term postoperative outcomes of patients undergoing surgical procedures immediately before vs after the weekend.

Design, Setting, and Participants  This is a population-based, retrospective cohort secondary analysis of adult patients in Ontario, Canada, undergoing 1 of 25 common surgical procedures between January 1, 2007, to December 31, 2019, with 1 year of follow-up. Data analysis was performed from October to November 2022.

Exposure  Undergoing surgery before (1 day before) vs after (1 day after) the weekend.

Main Outcomes and Measures  The primary outcome was a composite of death, readmission, and complications at 30 days, 90 days, and 1 year. Multivariable generalized estimating equations with an independent correlation structure, accounting for covariates, with clustering on surgical procedure were used to estimate the association between day of surgery in relation to the weekend and the outcomes.

Results  Of the 429 691 patients (mean [SD] age, 58.6 [16.9] years; 270 002 female patients [62.8%]) in the study cohort, 199 744 (46.5%) underwent surgery before the weekend, and 229 947 (53.5%) underwent surgery after the weekend. Patients in the preweekend group were more likely than those in the postweekend group to experience the composite outcome of death, complications, and readmissions at 30 days (adjusted odds ratio [aOR], 1.05; 95% CI, 1.02-1.08), 90 days (aOR, 1.06; 95% CI, 1.03-1.09), and 1 year (aOR, 1.05; 95% CI, 1.02-1.09) after surgery. Odds of mortality were increased in the preweekend group vs the postweekend group at 30 days (aOR, 1.09; 95% CI, 1.03-1.16), 90 days (aOR, 1.10; 95% CI, 1.03-1.17), and 1 year (aOR, 1.12; 95% CI, 1.08-1.17).

Conclusions and Relevance  In this retrospective multi-institutional study, patients who underwent surgery immediately preceding the weekend had a significantly increased risk of complications, readmissions, and mortality compared with those treated after the weekend. Further study is needed to understand differences in care that may underpin these observations and ensure that patients receive high-quality care regardless of the day of the week.