At all hospitals in the State of Iowa over a decade, the number of cases performed during weekends or holidays increased approximately proportionally to the total caseload.

Study Objective: We tested the hypothesis that over many years - a decade - hospitals' proportions of surgical cases that were performed on weekends and holidays remained stable.

Design: Retrospective cohort study.

Setting: Iowa Hospital Association data were from January 1, 2007, through June 30, 2017. The N = 42 hospitals included were those with at least 10 cases performed during holidays or weekends for each of the periods.

Measurements: The number of surgical cases performed at each hospital during each of the 21 half-year periods was considered the count of unique combinations of hospital, patient, and date with at least one major therapeutic procedure.

Main Results: Absolute predictive errors in cases per weekend or holiday day were calculated using a proportional model and using a quadratic model for each hospital and half-year period. Pooling among hospitals, the sample mean absolute predictive errors were greater for the proportional model than for the quadratic model (P < 0.0001). However, the mean difference was just 0.0027 cases per weekend or holiday day (SE 0.0001), significantly less than even 1 case per day (P < 0.0001). The sample means of the pairwise differences in predictive errors were smaller than 1 case per day for all 42 hospitals, significantly so for 41 of the 42 hospitals (P ≤ 0.005). These conditions applied to all other hospitals in the state, because each performed few cases on weekends and holidays.

Conclusions: For the anesthesia group caring for patients at a hospital over several years, weekend and holiday anesthesia caseload should be expected to increase approximately proportionately to changes during regular workdays. Average weekend workload can be benchmarked using hospitals' percentages of operating room cases performed on weekends and holidays.