Congress fixes the date
In 1939, President Franklin Delano Roosevelt broke with decades of tradition and attempted to move the Thanksgiving observance from the last Thursday in November to the second to last Thursday.
The move was widely criticized, with several states refusing to follow the new date for the major holiday. This prompted Congress to officially designate that the holiday be held on the fourth Thursday of November.
“Roosevelt, recognizing the problems caused by his 1939 decree, had announced a plan to return to the traditional Thanksgiving date in 1942,” explained the Library of Congress.
“But Congress introduced the legislation to ensure that future presidential proclamations could not affect the scheduling of the holiday. Their plan to designate the fourth Thursday of the month allowed Thanksgiving Day to fall on the last Thursday in five out of seven years.”