Illinois Administrative Code
Title 86 - REVENUE
Part 140 - SERVICE OCCUPATION TAX
Subpart K - TIMELY MAILING TREATED AS TIMELY FILING AND PAYING - MEANING OF DUE DATE WHICH FALLS ON SATURDAY, SUNDAY OR A HOLIDAY
Section 140.1101 - Filing of Documents with the Department

a) Any report, claim, tax return, statement or other document required or authorized to be filed with or any payment made to the Department of Revenue, which document or payment is transmitted through the United States mail, will be deemed to have been filed with and received by the Department on the date shown by the post office cancellation mark stamped upon the envelope or other appropriate wrapper containing it. If mailed but not received by the Department, or if received, but the cancellation mark is illegible, erroneous or omitted, the document or payment will be deemed to have been filed on the date it was mailed if the sender establishes by competent evidence that the document or payment was deposited in the United States mail on or before the date due for filing. If the envelope or other wrapper bears a postmark made by a private postage meter in addition to a legible postmark made by the United States Postal Service, the postmark not made by the United States Postal Service shall be disregarded. In the event of the Department's failure to receive a document or payment required by law to be filed, such document or payment will be deemed to have been received by the Department on time if the sender files with the Department a duplicate within 30 days after written notification is given to the sender by the Department of its failure to receive such document or payment, provided proof is furnished that the original of the document was deposited in the United States mail on or before the date due for filing.

b) If any report, claim, tax return, statement, remittance or other document is sent by United States registered mail, certified mail or certificate of mailing, a record authenticated by the United States Post Office of such registration, certification or certificate shall be considered competent evidence that the report, claim, tax return, statement, remittance or other document was mailed, and the date of registration, certification or certificate shall be deemed to be the postmarked date made by the United States Postal Service.

c) If the due date for any return or other report or payment falls on Saturday, Sunday or a Holiday, such due date shall be considered to be the next business day either for the purpose of submitting such return or other report or payment by mail or for the purpose of submitting such return or other report or payment in person.

d) Reports, claims, tax returns, statements, remittances or other documents delivered by means other than the United States mail are considered to be filed on the date they are received by the Department.