Hannibal Day served as an officer in the regular army before and during the American Civil War, briefly commanding a brigade in the Army of the Potomac.
Hannibal Day was born in Montpelier, Vermont on February 15, 1804. He attended the United States Military Academy at West Point, New York, graduating 23rd in a class of 35 in 1823. Day was assigned to the infantry, serving in Indian wars, including the Seminole War and in the Mexican War. Day was promoted to the rank of lieutenant colonel of the 2nd U.S. Infantry on February 25, 1861. He was serving at Fort Abercrombie in the Dakota Territory when the Civil War erupted. He was on recruiting duty in the early months of the conflict. During that time of service Day became colonel of the 6th U.S. Infantry on January 7, 1862.
Colonel Day joined the Army of the Potomac on June 28, 1863. He served as a brigade commander in the second division of V Corps under BG Romeyn B. Ayres at the Battle of Gettysburg. His brigade of regulars lost substantial casualties when it was attacked during deployment into the Wheatfield on July 2, 1863. The attack was executed by the brigade of Brig Gen William T. Wofford, and it cost Day's brigade 382 casualties. (Afterwards, Day's brigade was combined with that of Sidney Burbank, serving under Burbank in the Bristoe Campaign and the Mine Run Campaign. The regulars later were made part of a brigade under Ayers in General Charles Griffin's first division V Corps.)
Day was elderly, one of the oldest officers commanding at Gettysburg. (Only George S. Greene was older.) He left the Army of the Potomac on August 22, 1863 for less demanding assignments. He commanded Fort Hamilton in New York from August 1863 until June 8, 1864. He played minor administrative roles after that. A Brevet rank of brigadier general was awarded to him on March 13, 1865.
Day lived long after the war. He was one of the oldest graduates of West Point when he died, a widower, in Morristown, New Jersey on March 26, 1891. He was buried at Evergreen Cemetery in Leominster, Massachusetts.
- Pfanz, Gettysburg - the Second Day, pp. 297-301. Map 12-2 on p. 292.
- Boatner, Mark M., Civil War Dictionary, New York, D. McKay Co. . ISBN 0-8071-0882-7
- Eicher, John H., and Eicher, David J., Civil War High Commands, Stanford University Press, 2001, ISBN 0-8047-3641-3.
- Pfanz, Harry W., Gettysburg – The Second Day, University of North Carolina Press, 1987, ISBN 0-8078-1749-X. (Photograph of Burbank on p. 299.)
- Reese, Timothy J., Sykes' Regular Division 1861-1864, Jefferson, NC: McFarland, 1990. ISBN 0-89950-447-7