15 ILCS 405/13
The comptroller shall prepare a schedule showing the dates on which all employees of the State shall be paid. All employees shall be paid at least semi-monthly, except that employees of the University of Illinois, Chicago State University, Eastern Illinois University, Governors State University, Illinois State University, Northeastern Illinois University, Northern Illinois University, Western Illinois University, and Southern Illinois University who are not subject to the State Universities Civil Service System shall be paid at least once each month. No payment shall be made before the completion of the period for which the compensation is being paid, except that employees leaving the service of the State may be paid at the termination of their period of employment.
Such schedule shall be prepared showing such dates of payment so as to provide as far as is practical, an even flow of work for issuance of warrants in payment of personal services.
In making payments for a fractional part of a pay period, that part of the regular compensation for the period shall be paid represented by a fraction, the numerator being the number of days worked and the denominator being the number of work days in the period. However, in making payments for a fractional part of a pay period for positions subject to the jurisdiction of the Department of Central Management Services, that part of the regular compensation shall be paid by deducting an amount determined by multiplying the number of work days without pay by the applicable daily rate as defined within the Department of Central Management Services Pay Plan.
In employments of a teaching or professional nature in connection with the educational, charitable, penal or reformatory institutions, where the compensation of an employee is based upon an annual salary, a teaching or professional year may be substituted for a calendar year in determining the pay schedule.
15 ILCS 405/13