The Dakota Territorial Legislature created DayCounty 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 MarshallCounty, on the east by RobertsCounty, on the southeast by Grant and Codington counties, on the south by ClarkCounty, on the southwest by SpinkCounty and on the west by BrownCounty.  There are nine incorporated municipalities within DayCounty:  Andover, Bristol, Butler, Grenville, Lily, Pierpont, Roslyn, Waubay and Webster.