She is the one who convinced Abraham Lincoln to declare it a national holiday.
Sarah Josepha Hale, a 74-year-old magazine editor, wrote a letter to Lincoln on September 28, 1863, urging him to have the "day of our annual Thanksgiving made a National and fixed Union Festival." She explained, "You may have observed that, for some years past, there has been an increasing interest felt in our land to have the Thanksgiving held on the same day, in all the States; it now needs National recognition and authoritive fixation, only, to become permanently, an American custom and institution."Before the proclamation, Thanksgiving was held at different times in different states or communities as a general harvest festival.
You can read more about how the holiday started and President Lincoln's full proclamation at Abraham Lincoln Online.
Lets not forget Sarah Josepha Hale's contribution. Say some thanks to her this Thanksgiving Day for her contribution to modern American Culture.