The Dakota Territorial Legislature created Day County in 1880. The county was named in honor of Merritt H. Day, a pioneer and 1879 Dakota Territory legislator. In the November 1886 election Webster was officially named the county seat. The old courthouse was completed in 1883, and replaced by a new building in 1903. The current courthouse was constructed in 1963 to replace the old building which was destroyed by a fire in 1962.
Day County lies in northeastern South Dakota. It is bounded on the north by Marshall County, on the east by Roberts County, on the southeast by Grant and Codington counties, on the south by Clark County, on the southwest by Spink County and on the west by Brown County. There are nine incorporated municipalities within Day County: Andover, Bristol, Butler, Grenville, Lily, Pierpont, Roslyn, Waubay and Webster.