65 ILCS 5/5-2-6

A redistricting ordinance which has decreased the number of wards of a city because of a decrease in population of the city shall not be effective if, not less than 30 days prior to the first date fixed by law for the filing of candidate petitions for the next succeeding election for city officers, a specified census is officially published which shows that the city has regained a population that entitles it to the number of wards which it had just prior to the passage of the last redistricting ordinance.