Hannibal Day (February 15, 1804 – March 26, 1891) served as an officer in the United States Army.

[3] Day served at Fort Brady, Michigan from 1823 to 1828 and on surveying and topographical duty in the Western United States from 1828 to 1831.

[3] From 1833 to 1836, Day was on duty at Hancock Barracks, Maine and he served at Fort Independence, Massachusetts in 1836.

[3] Day served at Jefferson Barracks, Missouri in 1854, then performed frontier duty at Fort Ridgely, Minnesota from 1854 to 1855.

[3] He served as commander of First Brigade, Second Division, V Corps under Brigadier General Romeyn B. Ayres, which included combat at the Battle of Gettysburg.

[5] They were later made part of a brigade under Ayers in General Charles Griffin's First Division, V Corps.

[5] Day was one of the oldest officers commanding at Gettysburg; only his classmate George S. Greene was older.

[3] He served on military commissions and courts martial from July 25, 1864 until his final retirement on June 15, 1869.

[3] On March 13, 1865, he received promotion to the brevet rank of brigadier general in recognition of his many years of service.