From 1833 to 1836, Day was on duty at Hancock Barracks, Maine and he served at Fort Independence, Massachusetts in 1836. He served on recruiting duty at various posts from 1836 to 1838 and took part in the Seminole War of 1838-1839. Day became ill in Florida and was on convalescent leave from 1839 to 1841. He returned to Florida in 1841 and remained until 1842. He served with the 2nd Infantry in Buffalo, New York from 1842 to 1845 and Detroit, Michigan from 1845 to 1846. He took part in the Mexican-American War from 1846 to 1847, serving with the 2nd Infantry as part of David E. Twiggs' Brigade and participating in battles including Veracruz, Cerro Gordo, Contreras, Churubusco, Moline del Rey and Chapultepec.