40 ILCS 5/16-189.1

Retirement annuities and other benefits, unless otherwise specified in this Article, shall be paid in 12 monthly installments as of the first day of each month and shall cover the preceding month or proportionate part thereof then due. Provided, however, upon the death of a member in receipt of a benefit, an annuitant or a beneficiary, benefits shall be paid through the last day of the month in which death occurs.