S.D. Codified Laws § 10-59-32.1
Any return, report, or remittance which is required to be filed under the taxes specified in chapters 10-33A, 10-45, 10-45D, 10-46, 10-46A, 10-46B, 10-46E, 10-52, 10-52A, and 10-62, is timely filed if mailed, postage prepaid, on or before the due date of the reporting period, and is received by the department. If the due date falls on a Saturday, Sunday, legal holiday enumerated in § 1-5-1, or a day the Federal Reserve Bank is closed, the return, report, or remittance is timely filed if mailed, postage prepaid, on the next succeeding day which is not a Saturday, Sunday, legal holiday enumerated in § 1-5-1, or a day the Federal Reserve Bank is closed. A United States Postal Service postmark is evidence of the date of mailing for the purpose of timely filing of returns, reports, or remittances. The provisions of this section do not apply to a return filed by electronic means.
SDCL 10-59-32.1