The author of Mary Had a Little Lamb, Sarah Josepha Hale, is most responsible for the creation of Thanksgiving being a national holiday.
Sarah Josepha Buell Hale (October 24, 1788 – April 30, 1879) was an American writer and an influential editor. She was the author of the nursery rhyme "Mary Had a Little Lamb". Hale famously campaigned for the creation of the American holiday known as Thanksgiving, and for the completion of the Bunker Hill Monument.
Hale may be the individual most responsible for making Thanksgiving a national holiday in the United States; it had previously been celebrated mostly in New England. Each state scheduled its holiday, some as early as October and others as late as January; it was largely unknown in the American South. Her advocacy for the national holiday began in 1846 and lasted 17 years before it was successful.
In support of the proposed national holiday, Hale wrote presidents Taylor, Fillmore, Pierce, Buchanan, and Lincoln. Her initial letters failed to persuade, but the letter she wrote to Lincoln convinced him to support legislation establishing a national holiday of Thanksgiving in 1863. The new national holiday was considered a unifying day after the stress of the Civil War. Before Thanksgiving's addition, the only national holidays celebrated in the United States were Washington's Birthday and Independence Day.