Thanksgiving is a federal holiday in the United States celebrated on the fourth Thursday of November (which became the uniform date country-wide in 1941). The earliest Thanksgiving can occur is November 22; the latest is November 28. Outside the United States, it is called American Thanksgiving to distinguish it from the Canadian holiday of the same name and related celebrations in other regions. As the name implies, the holiday generally revolves around giving thanks and the centerpiece of most celebrations is a Thanksgiving dinner with family and friends.
The modern national celebration dates to 1863; prior to this, it was a regional holiday, whose origins lie in the 17th and 18th century days of thanksgiving of Calvinist New England. The evolution of the holiday was not linear (various New England communities had independently developed their own similar traditions that slowly turned into a singular annual Thanksgiving Day); the first known civil day of thanksgiving in the New England tradition was declared at Plymouth Colony in 1623, two years after the famous 1621 harvest celebration popularized as the "first Thanksgiving" bearing a substantial, if a coincidental, similarity to what Thanksgiving Day would eventually become. Celebrations of Thanksgiving for the harvest in New England became a regular occurrence by the 1660s.
