65 ILCS 5/6-3-8
An alderperson may resign from his or her office. A vacancy occurs in the office of alderperson by reason of resignation, failure to elect or qualify, death, permanent physical or mental disability, conviction of a disqualifying crime, abandonment of office, or removal from office. If a vacancy occurs in the office of alderperson in one of these ways or otherwise, the vacancy shall be filled as provided in Sections 3.1-10-50 and 3.1-10-55. An appointment to fill a vacancy shall be made within 60 days after the vacancy occurs. The requirement that an appointment be made within 60 days is an exclusive power and function of the State and is a denial and limitation under Article VII, Section 6, subsection (h) of the Illinois Constitution of the power of a home rule municipality to require that an appointment be made within a different period after the vacancy occurs.
65 ILCS 5/6-3-8