A nice job on this, but who was this guy?
Gettysburg, as we all know, was a battle absolutely loaded with moments of incredible bravery, sacrifice and drama, and Day's regiment provided one of the best,along with one of the most memorable quotes.
The two brigades of regulars, under Day and (I can't remember who but one of the GBurg experts surely does) were getting absolutely plastered in The Wheatfield when, finally, Sykes line broke under pressure from Semmes, Wofford, et al.
It was time to get out.
But rather than run for it or even just filter away in small groups, Day's regiment of US regulars formed up and marched out in formation under a galling fire, stopping from time to time to form firing lines, loosing a volley and then reforming but never breaking.
One of the volunteers watching this display from Little Round Top is quoted as saying:
"For two years, the regulars taught us how to fight like soldiers. At the Wheatfield, they taught us how to die like soldiers."
Also as a side note:
Day was a West Pointer who was on active duty when the war broke out. Although he was only a colonel - he was brevetted a Brigadier in March of 1865 when the war was pretty much over (although apparently in plenty of time to go get a uniform and have a photo taken; one suspects that this photo was taken after the war ) - in real terms he outranked most of the Major Generals in the Army.
This was because his rank was a permanent one, not of volunteer troops like most everyone else. When the war ended the vast majority of the general officers and colonels, if they were staying in the army, had to revert to their permanent rank.
Thus, a permanent rank of Colonel, which was Day's due to his posting, was highly coveted.