It was during the 19th century that Thanksgiving became a nationalized holiday. President Lincoln made a proclamation (Oct. 3, 1863) to that effect. Prior to 1863 states decided when or if they would celebrate thanksgiving. Setting the date as the last Thursday in Nov for all states to abide by.
Below is a link to the proclamation Lincoln wrote. Proclamation
This came up as I was researching the original thanksgiving with the Mayflower Pilgrims and thought some of you might enjoy this tidbit when writing your stories. You should research and see what some of the Southern states thought about Thanksgiving, very interesting.