Yet the first celebration probably happened somewhere between Sept. 21 and Nov. 11, likely not even on a Thursday. English harvest festivals on which the event was partly based occurred around Sept. 29 in those days [source: Mach].
While the celebration was commemorated in the years following the first Thanksgiving -- scaled down to a one-day affair -- Thanksgiving did not become a national holiday until 1863. That year, President Abraham Lincoln declared two national days of Thanksgiving: one on Aug. 6 to mark the Civil War Battle of Gettysburg, and the other on the last Thursday in November to commemorate the 1621 event. Magazine editor Sarah Josepha Hale is largely credited with lobbying Lincoln to establish the holiday, arguing that it would be a good way to unite a country torn apart by the ravages of war [source: Plimoth Plantation].
Succeeding presidents made Thanksgiving a once-a-year holiday each November, customarily falling on the last Thursday in November. In 1939, President Franklin D. Roosevelt moved the date up a week, setting Thanksgiving on the fourth Thursday of November to lengthen the Christmas shopping season. In 1941, Congress made it an official holiday, no longer requiring an annual presidential decree [source: Plimoth Plantation].