Credit for the establishment of Thanksgiving Day as a nation-wide holiday must go to Sarah Josepha Buell Hale, the editor of an influential women's magazine (and author of "Mary Had a Little Lamb") who lobbied legislatures and presidents from 1827 on. In 1863 Abraham Lincoln proclaimed the last Thursday of November as a day of "thanksgiving and praise to our beneficent Father who dwelleth in the Heavens," and since then it has been an annual celebration, though the date has varied. From 1939-1941 President Franklin Delano Roosevelt, in response to the complaints of businessmen that there was insufficient shopping time between Thanksgiving and Christmas, proclaimed Thanksgiving to be the third Thursday in November. This, however, created conflicts with the dating of the holiday in many states which had their own Thanksgiving legislation, so Congress in 1941 passed a joint resolution decreeing that the observance should fall on the fourth Thursday of November.
Bowler, Jerry. "Thanksgiving." St. James Encyclopedia of Popular Culture. 5 vols. St. James Press, 2000. Reproduced in History Resource Center. Farmington Hills, MI: Gale. http://galenet.galegroup.com/servlet/HistRC?locID=lln_pcalc