From 365 DAYS OF AMAZING TRIVIA page a day calendar:
Who is considered to be the “Mother of Thanksgiving”?
Sarah Hale (1788–1879), a prominent editor and writer who was a big voice in the movement to make Thanksgiving a national holiday. The idea of a making it a federal holiday was not hers, but she wrote many articles and editorials championing it as a way to bring together a country divided by the Civil War. And then, in 1863, Hale wrote a letter urging President Lincoln to proclaim a national day of Thanksgiving on the last Thursday in November. It's not known for certain if Hale's letter was what finally swayed Lincoln, but within a week of her letter, the president issued an official proclamation and Thanksgiving became a nationally observed holiday.