The photos were found by a reader “browsing through” the library’s Civil War photo negatives, reports The Los Angeles Times. The date was 4 March 1865 — they inaugurated presidents in March in those days — and the scene looks appropriately wet and gray.
(As an aside: Franklin Roosevelt (1937) was the first president to be inaugurated in January. The change from March to January 20th was decreed by the 20th Amendment in 1933. It had the side benefit of putting the inaugural in a drier month, according to this science question of the week from NASA. It also shortened FDR’s first term by about six weeks — which ought to be a juicy trivia answer somewhere.)
One bummer about the new photos — alas, they don’t include images of Lincoln himself.
But the Library of Congress collection does include this groovy shot from Lincoln’s first inaugural, with the capitol dome under construction.