205 ILCS 675/5
A financial institution may charge and collect interest under a revolving credit plan on outstanding unpaid indebtedness in the borrower's account under the plan at such periodic percentage rate or rates as the agreement governing the plan provides or as established in the manner provided in the agreement governing the plan. If the agreement governing the revolving credit plan so provides, the periodic percentage rate or rates of interest under such plan may vary in accordance with a schedule or formula. Such periodic percentage rate or rates may vary from time to time as the rate determined in accordance with such schedule or formula varies and such periodic percentage rate or rates, as so varied, may be made applicable to all outstanding unpaid indebtedness under the plan on or after the effective date of such variation, including any such indebtedness arising out of purchases made or loans obtained prior to such variation in the periodic percentage rate or rates. If the applicable periodic percentage rate under the agreement governing the plan is other than daily, periodic interest may be calculated on an amount not in excess of the average of outstanding unpaid indebtedness for the applicable billing period, determined by dividing the total of the amounts of outstanding unpaid indebtedness for each day in the applicable billing period by the number of days in the billing period. If the applicable periodic percentage rate under the agreement governing the plan is monthly, a billing period shall be deemed to be a month or monthly if the last day of each billing period is on the same day of each month or does not vary by more than 4 days therefrom.
205 ILCS 675/5