65 ILCS 5/1-7-2
Whenever in this Code any provision thereof is based upon the number of inhabitants, the number of inhabitants of the municipality shall be determined by reference to the latest census taken by authority of the United States or this state, or of that municipality. It is the duty of the Secretary of State, upon the publication of any state or United States census or the certification of any municipal census referenced under Section 1-7-1 , to certify to each municipality the number of inhabitants, as shown by that census. In the event that a partial census is conducted pursuant to Section 1-7-1, the Secretary of State shall certify the total number of inhabitants of the municipality as the number reflected by the last complete census of the municipality adjusted by the net increase or decrease reflected by the partial census. And the several courts in this state shall take judicial notice of the population of any municipality, as the population appears from the latest federal, state, or municipal census so taken , certified, and adjusted .
65 ILCS 5/1-7-2