After the first Thanksgiving was celebrated in 1621, each of the 13 American colonies held an annual harvest festival and day of thanks. There was no set day for the holiday and it varied among the colonies.
In 1777, George Washington decreed a day of Thanksgiving to celebrate the British defeat during the Revolutionary War. He and subsequent presidents proclaimed Thanksgiving periodically over the next 85 years, although not every year and not on any particular date. In the early 1800's, these Presidential proclamations came to an end, and it wasn't until 1863 that the holiday resumed.
During the Civil War, President Lincoln declared a national day of Thanksgiving to occur on the last Thursday of November, and it has been observed annually ever since. In 1939, in an effort to stimulate the economy during the Great Depression, President Franklin Roosevelt tried to move the holiday to the third Thursday in November, giving stores more time to advertise Christmas merchandise. About half the states refused to change the date, and in 1941, Congress compromised by officially choosing the fourth Thursday in November.