Born on February 15th, 1804, Civil War Union Brevet Brigadier General. After graduating from the United States Military Academy in 1823 (with future Union Generals Lorenzo Thomas and George S. Greene), he was posted to the 2nd United States Infantry, a unit that he would be associated with nearly his entire career.

His military service in the long years before the Civil War exhibited the typical stations of a career soldier, serving garrison posted on the Great lakes, on the East Coast and in the Western Frontier. He directed portions of his unit in the 1832 Black Hawk War (where he saw no action) in the Florida Seminole wars of 1839 and 1842, and during the Mexican War. He was promoted to Captain in 1838, fifteen years after his graduation as a 2nd Lieutenant. He was promoted to Major in 1852, and from Sept. 4, 1859 through May 15, 1860, Major Day commanded Ft. Laramie.

Day was promoted to Lieutenant Colonel of the 2nd Infantry in February 1861. Less than a year later he was promoted to full Colonel of the 6th United States Infantry in January 1862.  As one of the oldest officers in the Army, he was typically performed staff and recruitment posts. However, events of June and July 1863 thrust him into his single combat command of the war, and his first combat field command since the Mexican War. When Army of the Potomac commander General Joseph Hooker resigned as its head in late June 1863, Fifth Corps commander Major General George G. Meade was detailed to replace him., Major General George Sykes took over command of the Fifth Corps.  Sykes' First Division was then taken over by Brigadier General Romeyn B. Ayers, and Colonel Hannibal Day was assigned to lead Ayers old First Brigade, made up of Regular Army troops.

Having led the brigade for only three days, and being wholly unfamiliar with his officers and men, Colonel Day was positioned on the Federal Left flank on the Second day of the Battle of Gettysburg (July 2, 1863). When General John S. Caldwell's Division was thrown into the maelstrom of the Wheatfield, Colonel Day's brigade was sent in support, ands ended up at the eastern edge of the Rose Woods. His horse was killed in the action there, and his men, while lying prone, were flanked when Confederate forces pushed back the brigade in his right. He led the Regulars in an orderly retreat, but sustained 382 casualties during his relatively brief time on the field. After regrouping near Little Round Top, the brigade saw no more action.  Less than a month later (August 1) Colonel Day retired on his own application after forty consecutive years of service. He spent the next year on volunteer court martial duty. On March 13, 1864 he received a brevet of Brigadier general, US Regular Army for "long and faithful services in the Army". The old soldier passed away in Morristown, New Jersey on March 26, 1891.