The first Thanksgiving was celebrated after the harvest 400 years ago this autumn, in 1621. The exact date is unknown. The feast was celebrated by the Pilgrims, the breakaway Puritan sect that founded Plymouth Plantation in Massachusetts in 1620, and it was joined by the neighboring Native American tribe, the Wampanoags.
For the Wampanoags and many other American Indians, the fourth Thursday in November is considered a day of mourning, not a day of celebration. Because while the Wampanoags did help the Pilgrims survive, their support was followed by years of a slow, unfolding genocide of their people and the taking of their land.
Though the holiday is ironic, enjoy your gathering with family and friends, and don't eat too much!