Final answer:
Abraham Lincoln declared Thanksgiving a national holiday in 1863, with the date later fixed by Congress in 1941 to the fourth Thursday in November.
Step-by-step explanation:
The president who declared Thanksgiving a national holiday was Abraham Lincoln. He made this declaration in 1863, during the American Civil War, as a way to unify the nation. It was not until President Franklin D. Roosevelt, however, that the date of Thanksgiving was fixed to the fourth Thursday in November. This was done through a joint resolution of Congress in 1941, prompted by a push from Roosevelt to have a set national date for the holiday, which prior to that had been celebrated on different days in different states. This nationally recognized holiday has been celebrated annually ever since Lincoln's proclamation.