S.D. Codified Laws § 10-59-33
Any return, report, or remittance which is required to be filed under the taxes specified in § 10-59-1, except as provided for in §§ 10-59-32 and 10-59-32.1 and chapter 10-47B, 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.
SDCL 10-59-33