The tradition of the first Thanksgiving is steeped in legend. Few people realise that the Pilgrims did not celebrate Thanksgiving the next year, or any year thereafter, though some of their descendants later made a "Forefather's Day" that usually occurred on December 21 or 22. Several Presidents, including George Washington, made one-time Thanksgiving holidays. In 1827, Mrs. Sarah Josepha Hale began lobbying several Presidents for the instatement of Thanksgiving as a national holiday, but her lobbying was unsuccessful until 1863, when Abraham Lincoln finally made it a national holiday with his 1863 Thanksgiving Proclamation.

Today, Thanksgiving is the fourth Thursday of November. This was set by President Franklin D. Roosevelt in 1939 (approved by Congress in 1941), who changed it from Abraham Lincoln's designation as the last Thursday in November. But the Pilgrims' first Thanksgiving began at some unknown date between September 21 and November 9, most likely in very early October. The date of Thanksgiving was probably set by Lincoln to somewhat correlate with the anchoring of the Mayflower at Cape Cod, which occurred on November 21, 1620 .

There are only two contemporary accounts of the 1621 Thanksgiving: First is Edward Winslow's account, which he wrote in a letter dated December 12, 1621. The complete letter was first published in 1622, and is chapter 6 of Mourt's Relation: A Journal of the Pilgrims at Plymouth.

"Our corn [i.e. wheat] did prove well, and God be praised, we had a good increase of Indian corn, and our barley indifferent good, but our peas not worth the gathering, for we feared they were too late sown. They came up very well, and blossomed, but the sun parched them in the blossom. Our harvest being gotten in, our governor sent four men on fowling, that so we might after a special manner rejoice together after we had gathered the fruit of our labors. They four in one day killed as much fowl as, with a little help beside, served the company almost a week. At which time, amongst other recreations, we exercised our arms, many of the Indians coming amongst us, and among the rest their greatest king Massasoit, with some ninety men, whom for three days we entertained and feasted, and they went out and killed five deer, which they brought to the plantation and bestowed on our governor, and upon the captain and others. And although it be not always so plentiful as it was at this time with us, yet by the goodness of God, we are so far from want that we often wish you partakers of our plenty."