The Dayville Historic District encompasses a collection of mid-19th century architecture in the Dayville village of Killingly, Connecticut. It is clustered around the junction of Main and Pleasant Streets, extending along Main to High Street. The area flourished in the mid-19th century, as a consequence of the railroad being routed nearby, serving area textile mills. The district, residential except for a church, was added to the National Register of Historic Places in 1988.The village of Dayville was named after Capt. John Day, who owned the water privileges on the Five Mile River. With the opening of the Norwich and Worcester Railroad in 1830, Dayville also became a hub for freight transport for other manufacturing villages along the river. By 1849, the village had grown sufficiently in population to have its own church, the Dayville Congregational Church, a Greek revival building built by Alexander Blanchard.