Congress Makes It Official
After the controversy over President Franklin D. Roosevelt's moving of Thanksgiving, Congress passed a resolution in 1941 fixing the date for the observance.
"The Federal Thanksgiving Day holiday was established in 1941 following the passage of a joint resolution, H.J. Res. 41, by the House of Representatives on October 6, 1941, declaring the 'last Thursday in November a legal holiday,'" noted a Library of Congress website entry from 2010.
"The Senate then passed an amendment 'making the fourth Thursday in November a legal holiday.' President Franklin D. Roosevelt signed the resolution on December 6, 1941, with the law taking effect from 1942."