John H. Day was a resident of West Hartford on May 22, 1861 when he enlisted as a Private in Co. A of the 1st Regiment Connecticut Volunteer Heavy Artillery; he reenlisted as a veteran on  November 16, 1863, was promoted to Corporal September 16, 1865 and was mustered out September 25, 1865.

He was born about September 1839 in Lyme, Connecticut to John and Lydia (Latham) Day. He was not living in Mansfield in the 1860 U.S. census. After the Civil War, he moved to Mansfield and was a farmer. John Day married Emily E. Starkweather about 1868. Children include: Henry S. Day, born about March 1871; Daisy Day, born about 1872 and Johnnie Day, born about 1880.

On October 7, 1892, he applied for an invalid pension, No. 931,789 that was granted under certificate No. 870,455. His widow applied for a pension on January 11, 1902.

John H. Day died on December 31, 1901 in Mansfield and he is buried at the New Storrs Cemetery in Mansfield.