The fourth Thursday of November, America's feast of national origin, standardized by a novelist who wrote Mary Had a Little Lamb.
Documented
The proclamation
European thanksgiving services in North America predate Plymouth: Newfoundland 1578, Berkeley Hundred in Virginia by charter in 1619, Plymouth's own day of prayer in 1623. Washington proclaimed a national day in 1789; Jefferson refused on church-state grounds. Lincoln's October 3, 1863 proclamation made it annual, and the fourth Thursday was later fixed in law.
Assessment: Documented: proclamations, charters, and statutes are all on file.
Stories
The 1621 retrofit
New Englanders in the 1830s looked back at the Pilgrims' harvest feast and began calling it the First Thanksgiving because it resembled their own holiday. Hale, editor of Godey's Lady's Book, campaigned from the 1830s and 40s, writing Taylor, Fillmore, Pierce, Buchanan, and finally Lincoln, who answered within a week of her September 1863 letter.
Assessment: Stories: the continuity between 1621 and your table is a 19th-century editorial project.
Interesting claims
The darker claim
A persistent counter-narrative ties official thanksgiving days to celebrations after massacres of Native peoples, including the 1637 Pequot war proclamations. Such proclamations existed; historians still dispute how directly they feed the modern holiday, whose documented lineage runs through harvest observances and the Hale-Lincoln channel.
Assessment: Interesting claims: both lineages are real; the fight is over which one the holiday carries.
Sources and further reading
Abraham Lincoln, Thanksgiving Proclamation, October 3, 1863, printed in Harper's Weekly, October 17, 1863
George Washington's proclamation of a day of public thanksgiving, October 3, 1789
Berkeley Hundred charter (1619), Virginia: the arrival day 'yearly and perpetually kept holy as a day of thanksgiving'
Smithsonian Institution, 'Thanksgiving: From Local Harvests to National Holiday': Newfoundland 1578 and the 1623 Plymouth day of prayer
National Park Service, 'The Woman Behind Thanksgiving': Sarah Josepha Hale's letters to five presidents, 1846-1863
Worth asking
One editor petitioned for decades to manufacture a national memory and won; which of your family traditions was someone's campaign first?