[2] The county is named for Merritt H. Day, pioneer and 1879 Dakota Territory legislator.

[3][4] The terrain of Day County consists of rolling hills, partly devoted to agriculture.

[5] The terrain slopes to the west; its highest point is the northeast corner, at 2,014 ft (614 m) ASL.

Day County voters have tended to vote Democratic for the past several decades.

Since 1948 the county has selected the Democratic Party candidate in 71% of national elections.