In the United States, the modern Thanksgiving holiday tradition is commonly, but not universally, traced to a poorly documented 1621 celebration at Plymouth in present-day Massachusetts. The 1621 Plymouth feast and thanksgiving was prompted by a good harvest. Pilgrims and Puritans who began emigrating from England in the 1620s and 1630s carried the tradition of Days of Fasting and Days of Thanksgiving with them to New England. Several days of Thanksgiving were held in early New England history that have been identified as the "First Thanksgiving", including Pilgrim holidays in Plymouth in 1621 and 1623, and a Puritan holiday in Boston in 1631. According to historian Jeremy Bangs, director of the Leiden American Pilgrim Museum, the Pilgrims may have been influenced by watching the annual services of Thanksgiving for the relief of the siege of Leiden in 1574, while they were staying in Leiden. In later years, religious thanksgiving services were declared by civil leaders such as Governor Bradford, who planned a thanksgiving celebration and fast in 1623. The practice of holding an annual harvest festival did not become a regular affair in New England until the late 1660s.

Thanksgiving proclamations were made mostly by church leaders in New England up until 1682, and then by both state and church leaders until after the American Revolution. During the revolutionary period, political influences affected the issuance of Thanksgiving proclamations. Various proclamations were made by royal governors, John Hancock, General George Washington, and the Continental Congress, each giving thanks to God for events favorable to their causes. As President of the United States, George Washington proclaimed the first nation-wide thanksgiving celebration in America marking November 26, 1789, "as a day of public thanksgiving and prayer to be observed by acknowledging with grateful hearts the many and signal favours of Almighty God". [FROM WIKIPEDIA - THANKSGIVING U.S.]