On October 3, 1863, expressing gratitude for a pivotal Union Army
victory at Gettysburg, President Abraham Lincoln announced that
the nation will celebrate an official Thanksgiving holiday on
November 26, 1863.

The speech, which was actually written by Secretary of State
William Seward, declared that the fourth Thursday of every
November thereafter would be considered an official U.S.
holiday of Thanksgiving.

William Henry Seward
(May 16, 1801 – October 10, 1872)