5 ILCS 70/1.11

The time within which any act provided by law is to be done shall be computed by excluding the first day and including the last, unless the last day is Saturday or Sunday or is a holiday as defined or fixed in any statute now or hereafter in force in this State, and then it shall also be excluded. If the day succeeding such Saturday, Sunday or holiday is also a holiday or a Saturday or Sunday then such succeeding day shall also be excluded.