For 122 years, the California Labor Code has said that employees in all industries are “entitled” to a day of rest “one day therefrom in seven.” The statute also provides that “No employer shall cause his employees to work more than six days in seven.” Mendoza, a former Nordstrom employee, is arguing to the California Supreme Court that the Labor Code should be construed as flatly prohibiting employers from allowing an employee to work on the seventh day of a workweek. To make that argument work he must also convince the Court that the Labor Code prohibits employees from voluntarily choosing to work on a day otherwise scheduled for rest. This radically paternalistic argument not only flies in the face of the plain language of the statute, but it would hurt employees who may wish work on the seventh day of a workweek for innumerable reasons. In a brief filed in support of Nordstrom, Cato, joined by the National Federation of Independent Business, the Reason Foundation and a handful of California employees, argues that there are many legitimate reasons why an employee might want to work on the seventh day of a workweek: to meet financial goals, to accommodate personal schedules, or simply to maintain flexibility to work when he wants. Mendoza also argues that employers must require written waiver from employees before allowing them to work on the seventh day of a workweek. But nothing in the Labor Code suggests that there is any requirement for a waiver to be in writing, or for employers to maintain records whenever an employee should elect to work on a day otherwise scheduled for rest. We argue that it would be improper to read language into the statute that would impose such burdensome requirements on employers—both because it would violate first principles of statutory construction and because it would open unwitting businesses up to lawsuits. Moreover, such a paperwork requirement would be wholly impracticable when, for example, an exempt employee might choose to check a few emails on a Sunday evening, something that could be construed to violate the day-of-rest law. Finally, we argue that the plaintiff advocates a theory that would hold his employer liable for conduct that California state regulators had long permitted in official agency guidance. Just as there would be significant due process concerns with Congress passing a statute to retroactively hold businesses liable for conduct that was permissible at the time, there would be serious constitutional problems with giving a statute a retroactive interpretation that would impose ruinous penalties on individuals or businesses that acted in good faith reliance on best available guidance at the time. The California Supreme Court should not heed Mendoza’s paternalistic arguments and upset 122 years of treating California’s workers like responsible adults.