The quick answer to the oft repeated question, "When is Thanksgiving this year?", is simple: It's the fourth Thursday in November. It hasn't always been that way, however.
The first Thanksgiving feast. The History Channel asserts that in 1621 "the Plymouth colonists and Wampanoag Indians shared an autumn harvest feast that is acknowledged today as one of the first Thanksgiving celebrations in the colonies." There is no indication the feast was held on the fourth Thursday in November, nor was it necessarily held on the fourth Thursday in November during the next two-plus centuries in which colonies held Thanksgiving feasts in the fall.
State celebrations. Although presidents Washington, Adams and Monroe declared official days of thanks during their presidency, it was not an official state or national holiday. In 1817, New York became the first of several states to proclaim Thanksgiving as an official holiday. These states, however, celebrated it on different days. The South remained largely unaware of the tradition.
Sarah Josepha Hale's contribution. Although turkey is the center of the traditional Thanksgiving feast, it was a woman associated with lambs that played a major role in making Thanksgiving a national holiday. In 1827, Sara Josepha Hale, author of "Mary Had a Little Lamb," launched a campaign to establish Thanksgiving as a national holiday. Her campaign consisted of 36 years of editorials and letters to governors, senators and presidents before Thanksgiving was finally adopted as a national holiday in 1863.
Presidential proclamations. The National Archives indicates that George Washington, on Oct. 3, 1789, proclaimed Thursday, Nov. 26, 1789, as an official holiday of "sincere and humble thanks." This celebration became the first Thanksgiving under the new constitution. On Oct. 3, 1863, President Lincoln made Thanksgiving a nationwide holiday to be celebrated on the fourth Thursday in November. Franklin Roosevelt changed the date in 1939 to the third Thursday in November to lengthen the holiday shopping season and give the economy a boost. This holiday move set off a fierce debate, and in 1941 Thanksgiving was re-established as the fourth Thursday.