In the semiconductor industry, dynamic changes in demand force companies to change the product mix frequently and periodically. Assigning tight but attainable due dates is a great challenge under the circumstances that the product mix changes periodically. In this paper, we consider the due-date assignment problem for wafer fabrication and present a due-date assignment model to set manufacturing due dates satisfying the target on-time-delivery rate. The contamination model is applied to tackle the effect of that product mix varies periodically. We demonstrate the effectiveness and accuracy of the proposed model by solving a real-world example taken from a wafer fabrication shop floor in an IC manufacturing factory.