815 ILCS 405/12.1: Final installment.

Fifteen days after the final installment is due as originally scheduled or deferred, the holder may compute and charge interest on any balance remaining unpaid, including unpaid default or deferment charges, at the annual percentage rate stated in the retail installment contract until fully paid or until reduced to judgment. At the time the final installment is due, the holder shall give notice to the buyer stating any amounts unpaid.