Periodic Returns. Quarter-monthly deposits with Form IL-501 within 3 banking days after quarter-monthly period if aggregate withholdings exceed $1,000; monthly deposits with Form IL-501 by 15th day of following month if aggregate withholdings are more than $500 but less than $1,000 at anytime during the quarter; quarterly returns on Form IL-941 by last day of month following end of quarter if more than $500 withheld annually (if less than $500 withheld per quarter, quarterly payment is optional); annually may be permitted if employer withholds less than $500 for year, on Form IL-941 with payment.

Annual Reconciliation Returns. Form IL-W-3 due by February 28.

Timely Filing. If the due date for filing returns and reports falls on a Saturday, Sunday, or legal holiday, the next succeeding day which is not a Saturday, Sunday, or legal holiday shall be the due date. A return will be considered timely filed if the postmark date is the day on which the return is due.