65 ILCS 5/4-3-1
All municipalities which have adopted the commission form of municipal government prior to August 15, 1941, shall elect a mayor and 4 commissioners at a general municipal election held in an odd numbered year upon the expiration of the term of office of the mayor or president, and every 4 years thereafter. The day when elections in these municipalities shall be held shall be determined by the provisions of the general election law.
In all municipalities which adopt this article on or after August 15, 1941, the terms of office of all elected municipal officers (1) holding office at the time this article is adopted, or (2) elected on the same day that this article is adopted, or (3) elected at the next biennial election held after the adoption, shall expire at the end of their respective terms but not later than the expiration of the term of office of the mayor or president who is holding office at the time of the adoption of this article or who is elected to the office of mayor or president on the same day that this article is adopted, as the case may be.
All municipalities which adopt this article on or after August 15, 1941, shall elect a mayor and 4 commissioners at a general municipal election held in an odd numbered year upon the expiration of the term of office of the mayor or president as provided in the next preceding paragraph, and every 4 years thereafter. The day when elections in these municipalities shall be held shall be determined by the provisions of the general election law.
65 ILCS 5/4-3-1