S.D. Codified Laws § 10-47B-190
Any entity who holds a license issued pursuant to this chapter or any entity subject to the provisions of this chapter shall file returns or reports by electronic means with the department and shall remit tax by electronic transfer to the department unless the secretary permits an entity to file returns or reports by nonelectronic means and permits an entity to remit tax by nonelectronic means. If the day the return or report is due falls on a Saturday, Sunday, legal holiday enumerated in § 1-5-1, or a day the Federal Reserve Bank is closed, the return or report is due on the next succeeding day that is not a Saturday, Sunday, legal holiday enumerated in § 1-5-1, or a day the Federal Reserve Bank is closed. All tax required to be remitted is due and payable by electronic transfer on or before the twenty-fifth day of the month following each period. 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 remittance is due on the next succeeding day that is not a Saturday, Sunday, legal holiday enumerated in § 1-5-1, or a day the Federal Reserve Bank is closed.
If the secretary permits any entity to file returns or reports by nonelectronic means, permits any entity to remit tax by nonelectronic means, or both, any return, report, or remittance which is required to be filed is timely filed if mailed, postage prepaid, on or before the due date for the particular reporting period, and is received by the department. The return or report to be filed shall be on forms prescribed and furnished by the department. If the day the return, report, or remittance is due 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 due on the next succeeding day that is not a Saturday, Sunday, legal holiday enumerated in § 1-5-1, or a day on which 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 10-47B-190