Del. Code tit. 19, § 1907

An employer must also give notice if the number of employment losses which occur during a 30-day period fails to meet the threshold requirement of a mass layoff or plant closing, but the number of employment losses of 2 or more groups of workers, each of which is less than the minimum number needed to trigger notice, reaches the threshold level during any 90-day period of a mass layoff, plant closing or relocation. Job losses within any 90-day period will count toward WARN Act threshold levels unless the employer demonstrates that the employment losses during the 90-day period are the result of separate and distinct actions and causes.