65 ILCS 20/21-39
No such redistricting ordinance shall take effect until the expiration of 15 days after its passage. If within such 15 days 1/5 or more of the alderpersons elected, who did not vote to pass such redistricting ordinance, file with the city clerk a proposed substitute ordinance redistricting the city in accordance with the provisions of this article, together with a petition signed by them demanding that the question of the adoption of the redistricting ordinance passed by the city council, together with the question of the adoption of such substitute ordinance, be submitted to the voters, then such redistricting ordinance passed by the city council shall not go into effect until the question of this adoption shall have been submitted to a popular vote: Provided, that no alderperson shall have the right to sign more than one such petition. Upon the expiration of such 15 days the city clerk shall promptly certify to the board of election commissioners of the city of Chicago, the ordinance passed by the city council and such substitute ordinance or ordinances and petition or petitions, and it shall thereupon be the duty of the board of election commissioners to submit the ordinances so certified to a popular vote at the next general or municipal election, to be held in and for the entire city not less than 40 days after the passage of such redistricting ordinance by the city council.
65 ILCS 20/21-39