The following is an article from history.com:
“On October 3, 1863, expressing gratitude for a pivotal Union Army victory at Gettysburg, President Abraham Lincoln announces 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 last Thursday of every November thereafter would be considered an official U.S. holiday of Thanksgiving. This announcement harkened back to when George Washington was in his first term as the first president in 1789 and the young American nation had only a few years earlier emerged from the American Revolution. At that time, George Washington called for an official celebratory “day of public thanksgiving and prayer.” While Congress overwhelmingly agreed to Washington’s suggestion, the holiday did not yet become an annual event.
Thomas Jefferson, the third president, felt that public demonstrations of piety to a higher power, like that celebrated at Thanksgiving, were inappropriate in a nation based in part on the separation of church and state. Subsequent presidents agreed with him. In fact, no official Thanksgiving proclamation was issued by any president between 1815 and the day Lincoln took the opportunity to thank the Union Army and God for a shift in the country’s fortunes on this day in 1863.
The last Thursday of November remained the annual day of Thanksgiving from 1863 until 1939. Then, at the tail-end of the Depression, President Franklin D. Roosevelt, hoping to boost the economy by providing shoppers and merchants a few extra days to conduct business between the Thanksgiving and Christmas holidays, moved Thanksgiving to November’s second-to-last Thursday. In 1941, however, Roosevelt bowed to Congress’ insistence that the last Thursday of November be re-set permanently, without alteration, as the official Thanksgiving holiday.”
