Really? I've lived in Illinois for all of my 55 years and never knew there to be such a law. Though I know that in certain towns they get all worked up about it,In Illinois the law says no operating of yard macinery before 8 am monday - friday and 9 am on the weekends. This law also limits to 9 pm. I have seen a few neighbors cutting grass as early as 7 but noone seems to mind up here in the rockford area. If you are in dekalb, they may be a little more criticle of the law due to the college.
but I've never known anyone to be ticketed for mowing outside any certain hours, usually it's certain neighbors that whine and complain among themselves.
I have one myself that used to, if I had anything running after 6pm.......
I remember pulling about a dozen mowers and GTs out in the driveway to fire them up all at once, letting them run at full throttle for "exercise" just because of that certain neighbor. And still do bring everything I have at the house at a given time out and fire them up to drown out a certain couple who live to scream and argue with each other.... I don't have as many any more though, as I used to.