Thanksgiving was always celebrated in America but became a national holiday by a Presidential Proclamation of Abraham Lincoln on October 3rd 1863 in the midst of the Civil War.
The original date for Thanksgiving Day was set for Thursday November 26th.
Later in 1942 President Franklin D. Roosevelt through a joint resolution in Congress issued a proclaimed designating the 4th Thursday in November to be Thanksgiving Day regardless of the numerical date.
Thanksgiving began as a strictly religious holiday but now it is primarily an opportunity for families to get together.