Wisconsin's Business Closing (Plant Closing) and Mass Layoff Law provides that, with certain exceptions, businesses employing 50 or more persons in the State of Wisconsin must give you written notice 60 days before implementing a "business closing (plant closing)" or "mass layoff" in the state.
A "business closing (plant closing)" requires notice if there is a permanent or temporary shutdown of an employment site or of one or more facilities or operating units within a single municipality that affects 25 or more employees (not including "new" or "low-hour" employees).
A "mass layoff" is at least 25% of the workforce or 25 employees, whichever is greater; or at least 500 employees.
Federal Business Closing Notification Law, the Worker Adjustment and Retraining Notification (WARN), helps ensure advance notice in cases of qualified plant closings and mass layoffs. The U.S. Department of Labor has a workers and employers guide to this law available at the link above.