40 ILCS 5/13-216
"Surviving spouse's annuity": The amount payable as a surviving spouse annuity commencing on the date of the employee's or retiree's death. The annuity shall be payable in equal monthly installments for life, except as otherwise provided in this Article, in the month after the effective date of the annuity . A pro rata amount of the annuity shall be paid for part of a month when the annuity begins after the first day of the month or ends before the last day of the month.
Notwithstanding the above, all surviving spouse annuity payments first payable on or after January 1, 2008, shall begin the first of the month following the employee's or annuitant's date of death.
Effective January 1, 2008, benefits are payable for the full month if the annuitant was alive on the first day of the month.
40 ILCS 5/13-216