Mass Layoff: A covered employer must give notice if there is to be a mass layoff which does not result from a plant closing, but which will result in an employment loss at the employment site during any 30-day period for 500 or more employees, or for 50-499 employees if they make up at least 33% of the employer's active workforce. Again, this does not count employees who have worked less than 6 months in the last 12 months or employees who work an average of less than 20 hours a week for that employer. These latter groups, however, are entitled to notice (discussed later).
This was lifted off of the govt web site.