“Thanksgiving may be a national holiday today, but originally it was a “Yankee” holiday. As New Englanders migrated to other areas of the country, they took the holiday with them, spreading it south and west.
President Abraham Lincoln declared Thanksgiving a national holiday in 1863 during the Civil War to promote national unity. Thanksgiving had already spread, but white Southerners still tended to see it as a Yankee holiday, especially after Lincoln’s proclamation.
So, after the Civil War, many white Southerners celebrated Thanksgiving on their own schedule or forwent the holiday altogether. Meanwhile, African Americans in the South embraced Thanksgiving, celebrating it along with the rest of the nation.
It was not until the opening decade of the 20th century that white Southerners fully adopted the holiday as well. By the time President Franklin Roosevelt signed a bill permanently placing Thanksgiving on the fourth Thursday of November, it had been incorporated as a holiday celebrating family into the lives of most Americans.”
–from “Thanksgiving and African-American History” by Lisa Vox