There are 7 US Post Offices in Day County serving a total of 5,722 residents equating to 817 residents per post office. It's estimated that approximately 9,133 packages are handled across the 7 county post offices annually. Day County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 5,710. Its county seat is Webster. The county is named after Merritt H. Day, pioneer and 1879 Dakota Territory legislator."