Answer :

demitube2014
Good question. 
To put it simply, it is the early American settler's way to celebrate the first successful harvest they had since they arrived from europe.
jelenako57605
Thanksgiving is celebrated in the United States as a national holiday dedicated to being with family and friends to give thanks for all of the blessings received throughout the previous year. The holiday is believed to have been celebrated first in 1621 when the Plymouth settlers enjoyed a fall feast to celebrate a bountiful harvest with the local Wampanoag Indians.

Hope this helps!! :D

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