S.D. Codified Laws § 12-4-5
The county auditor shall enter in the master registration file the name of each eligible person whose completed application for registration and mail registration card is received no later than 5:00 p.m. local time at least fifteen days preceding the election by the county auditor or the local, state, or federal agency responsible for conducting voter registration under this chapter. A voter registration completed at any local, state, or federal agency during any week commencing on Tuesday through the following Monday shall be sent to the appropriate county auditor by the agency receiving the registration or mail registration card no later than the following Wednesday. The State Board of Elections may promulgate rules, pursuant to chapter 1-26, for the alternative transmission of voter registration information by computer from the agency to the secretary of state. The name of any voter who has registered to vote by 5:00 p.m. local time fifteen days preceding a runoff election shall be added to the file used for the runoff election.
SDCL 12-4-5