Even though President Washington chose that date for the first “national day of thanksgiving celebration”, the holiday has been celebrated on numerous dates throughout the years and there were different days celebrated by the individual colonies and states. However, the final Thursday in November was the usual date to celebrate by the time the 19th century rolled around although it did not become “official” for all the states in the Union in the year 1863 by President Lincoln. He wrote up a proclamation making it fact.
One reason Lincoln wanted to do it was he wanted to try and give a sense of unity between the Northern and Southern states. The Civil War was going on at this time, so the Confederate States of America’s did not pay attention to President Lincoln’s power
This caused a nationwide Thanksgiving date to not be recognized until the Reconstruction was finalized in the late 1870’s.
It was on the 26th of Dec. 1941 that President Franklin D. Roosevelt signed a resolution that came from Congress which changed when the United States celebrated Thanksgiving Day. It was moved from the last Thursday to the fourth Thursday in November. It had been two years previous that President Roosevelt decided to use a presidential proclamation to attempt to do this change because he felt it would give the country some sort of boost in economics. [Source]