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Thanksgiving is a national holiday in the United States, celebrated on the fourth Thursday of November. It originated as a harvest festival. Thanksgiving has been celebrated nationally on and off since 1789, with a proclamation by George Washington after a request by Congress. Thomas Jefferson chose not to observe the holiday, and its celebration was intermittent until the presidency of Abraham Lincoln, when Thanksgiving became a federal holiday in 1863, during the American Civil War. Lincoln proclaimed a national day of Thanksgiving to be celebrated on the last Thursday in November. Under President Franklin D. Roosevelt, the date was changed to the fourth Thursday in November, an innovation that endures to this day. Together with Christmas and the New Year, Thanksgiving is a part of the broader fall–winter holiday season in the U.S.

The event that Americans commonly call the "First Thanksgiving" was celebrated by the Pilgrims after their first harvest in the New World in October 1621. This feast lasted three days, and—as accounted by attendee Edward Winslow—it was attended by 90 Native Americans and 53 Pilgrims. 

Thanksgiving Dinner
The centerpiece of contemporary Thanksgiving in the United States and Canada is a large meal, generally centered on a large roasted turkey. It is served with a variety of side dishes which vary from traditional dishes such as mashed potatoes, stuffing, and cranberry sauce, to ones that reflect regional or cultural heritage. The majority of the dishes in the traditional American version of Thanksgiving dinner are made from foods native to the New World, as according to tradition the Pilgrims received these foods, or learned how to grow them, from the Native Americans. Thanksgiving dinner is the largest eating event in the United States; people eat more on Thanksgiving than on any other day of the year.

Source: Wikipedia