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. 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. 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.”