If a due date falls on a Saturday, Sunday or legal holiday, the due date is extended until the next business day. Returns mailed through the U.S. mail shall be deemed filed and received by the department on the date shown by the post office cancellation mark on the envelope. If the post office cancellation mark is illegible, erroneous or omitted, the date received stamped by the department will be used to determine if the return was timely filed.
Note: Those taxpayers required to make payment in immediately available funds must do so in a manner that makes the funds immediately available to the state no later than the due date. In regard to an ACH credit payments, the taxpayer needs to initiate the ACH transaction one business day before the due date of the filing period to make sure payment is received by the due date.
Reference: Tenn. Code Ann. § 67-1-114