Thanksgiving Day started with the Pilgrims who came to America in 1620. They had a big feast with the Native Americans to celebrate their first successful harvest. This event is often seen as the first Thanksgiving.
Over time, Thanksgiving became a national holiday in the United States. Every year, people celebrate it with a big meal, usually including turkey, to give thanks for what they have.
Resource Link: https://americanheritage.org/the-history-of-thanksgiving-day-in-America/