Fact: The original feast in 1621 occurred sometime between September 21 and November 11. Unlike our modern holiday, it was three days long. The event was based on English harvest festivals, which traditionally occurred around the 29th of September. After that first harvest was completed by the Plymouth colonists, Gov. William Bradford proclaimed a day of thanksgiving and prayer, shared by all the colonists and neighboring Indians. In 1623 a day of fasting and prayer during a period of drought was changed to one of thanksgiving because the rain came during the prayers. Gradually the custom prevailed in New England of annually celebrating thanksgiving after the harvest.
During the American Revolution a yearly day of national thanksgiving was suggested by the Continental Congress. In 1817 New York State adopted Thanksgiving Day as an annual custom, and by the middle of the 19th century many other states had done the same. In 1863 President Abraham Lincoln appointed a day of thanksgiving as the last Thursday in November, which he may have correlated it with the November 21, 1621, anchoring of the Mayflower at Cape Cod. Since then, each president has issued a Thanksgiving Day proclamation. President Franklin D. Roosevelt set the date for Thanksgiving to the fourth Thursday of November in 1939 (approved by Congress in 1941.)