Much of the credit for the adoption of a later ANNUAL national Thanksgiving Day may be attributed to Mrs. Sarah Joseph Hale, the editor of Godey’s Lady’s Book. For thirty years, she promoted the idea of a national Thanksgiving Day, contacting President after President until President Abraham Lincoln responded in 1863 by setting aside the last Thursday of November as a national Day of Thanksgiving. Over the next seventy-five years, Presidents followed Lincoln’s precedent, annually declaring a national Thanksgiving Day. Then, in 1941, Congress permanently established the fourth Thursday of each November as a national holiday.
Lincoln’s original 1863 Thanksgiving Proclamation came - spiritually speaking - at a pivotal point in his life. During the first week of July of that year, the Battle of Gettysburg occurred, resulting in the loss of some 60,000 American lives. Four months later in November, Lincoln delivered his famous "Gettsysburg Address." It was while Lincoln was walking among the thousands of graves there at Gettysburg that he committed his life to Christ. As he explained to a friend:
“When I left Springfield [to assume the Presidency] I asked the people to pray for me. I was not a Christian. When I buried my son, the severest trial of my life, I was not a Christian. But when I went to Gettysburg and saw the graves of thousands of our soldiers, I then and there consecrated myself to Christ.”
Source: Christiananswers.net