40 ILCS 5/17-148
Annual pensions and automatic annual increases shall be paid in 12 monthly installments and shall be so adjusted that all monthly payments are the same. This shall apply also to automatic increases accrued from the anniversary of the pension or the 61st birthdate, whichever is later, to the following January. In computing the first pension payment for a fractional part of a month, 30 days shall constitute one month. Beginning January 1, 1970, the pension payment shall begin on the first day of the month, with the issuance of the last check on the first day of the month in which death occurs. If a pensioner is reinstated as a contributor, no pension shall be paid for the month in which re-employment occurs. The pensioner shall reimburse the Fund for any pension payments received to which he is not legally entitled, plus 5% interest compounded annually beginning one year after the Fund's notification of the indebtedness.
40 ILCS 5/17-148