65 ILCS 5/5-2-5

Whenever an official publication of any national, state, school, or city census shows that any city contains more or less wards than it is entitled to, the city council of the city, by ordinance, shall redistrict the city into as many wards only as the city is entitled. This redistricting shall be completed not less than 30 days before the first date fixed by law for the filing of candidate petitions for the next succeeding election for city officers. At this election there shall be elected the number of alderpersons to which the city is entitled.