I've seen this posted a bunch of places, as many states have a registration deadline today, and it's a bit confusing.
Today is the last day that you can register in person at the County Clerk's office or through motor voter registration at the DMV (or various other places). You can register to vote online through 10/21 (according to info I found through Rock the Vote and the New York Times — I could not find it anywhere on the County Clerk site or Illinois elections site).* You have until 4:30 p.m. today to register using one of those two methods.
HOWEVER. Tomorrow will begin Grace Period Voting, which extends through 7 p.m. on November 6th. You can go to any early voting location or to your polling place on election day and register in person, then immediately cast your ballot. So make sure that you are ready to do so if you utilize this option. You can look up your voter registration and see your sample ballot here.
Also, vote. Please.
*This was corrected from an earlier statement that said online registration closed today.