Computation of time shall begin with the first business day following the day on which the act, event or development initiating such period of time occurs. Time shall run until the end of the last day or the next following business day if the last day is a Saturday, Sunday or legal holiday. Where the period of time is five days or less, Saturdays, Sundays and legal holidays shall always be excluded in the computation of time.
Ill. Admin. Code tit. 35, § 168.240