820 ILCS 140/4

Before operating on the first day of the week, which is commonly known as Sunday, every employer shall post in a conspicuous place on the premises, a schedule containing a list of his employees who are required or allowed to work on Sunday, and designating the day of rest for each. Anything in this Act to the contrary notwithstanding, no employee shall be required to work on the day of rest so designated for him.