Objective: Study the variation in the number of accidents across days of the workweek, from Monday to Friday. Data and Method: All occupational accidents of full-time workers in Spain from 2011 to 2018 are analyzed. A total amount of 2,873,872 accidents are representative and not heavily affected by underreporting bias. Chi-squared test (χ2) and z-value determine statistically significant differences in accident percentages from Monday to Friday, controlling personal (sex, age, nationality, and seniority), company (activity, size, and region), temporal (hour of the day and working hour) variables, and consequences (severity, injury type, and injured body part). Results: “Weekday effect” with a decrease in accidents of 29.4% from Monday to Friday, a decrease of 14.3% from Monday to Tuesday (“Monday effect”), 3.9% from Tuesday to Wednesday, 7.9% from Wednesday to Thursday, and 7% from Thursday to Friday. The “weekday effect” and the “Monday effect” occur across industries, for all company sizes, for all types of workers (both genders, different ages and seniority), for different types of injury, and across the day and the shift. “Monday effect” increases with age and seniority, for men, in small companies (10 to 49 workers), in the morning, at the beginning of the work shift hours, and for back injuries. “Weekday effect” decreases with age and increases with seniority, for men, in companies between 20 and 249 workers, in the morning and at the beginning of the work shift hours, and for back injuries.
