Illinois (IL) automatically grants a 6-month extension to October 15. If you owe them taxes, submit payment by April 15 to avoid penalties. You can either mail a check along with Form IL-501-I or pay online at MyTax Illinois.
These extensions do not grant you an extension of time to pay any tax you owe. If you determine that you will owe tax, you must use Form IL-505-I, Automatic Extension Payment for Individuals, to pay any tax you owe to avoid penalty and interest on tax not paid by April 15, 2019.
MyTax Illinois is an easy way to pay your IL-505-I payment. If you do not expect to owe tax, you do not have to do anything.