35 ILCS 200/18-10
The county board of each county with less than 3,000,000 inhabitants shall, annually, at the September session, determine the amount of county taxes to be levied for all purposes. Any county with less than 3,000,000 inhabitants which has changed its fiscal year may, at the September session or at any adjourned meeting thereof, instead of determining the amount of all county taxes to be levied for a one-year period, determine the amount of taxes to be levied during a period greater or less than a year as required by the change of the fiscal year. The county board of each county with 3,000,000 or more inhabitants shall, annually, prior to the third Monday of March, determine the amount of county taxes to be levied for all purposes. The amount for each purpose shall be stated separately. All counties shall certify to the county clerk annually, on or before the last Tuesday in December the amounts that they have levied.
35 ILCS 200/18-10