In 1863 President Abraham Lincoln issued a Proclamation stating that Thanksgiving would be the last Thursday of November. Prior to Lincoln’s Proclamation, Thanksgiving celebrations varied from year to year with the dates and months constantly changing. Then in 1939, when Thanksgiving fell on the last day in November, President Franklin D. Roosevelt was concerned that a shortened Christmas shopping season would dampen the economic recovery. He issued a Presidential Proclamation changing the celebration to the second to last Thursday in November.
Not wanting to deviated from tradition, some states refused to move the date of celebration. For two years, the nation and some states celebrated Thanksgiving on the second to last Thursday of November while other states continued to celebrate on the last Thursday of the month.
To unite the nation and end confusion, Congress decided to fix the date of the holiday. On October 6, 1941, the House passed a joint resolution declaring that the last Thursday in November was a legal holiday. The Senate, however, amended the resolution establishing the holiday as the fourth Thursday, which would take into account those years when November has five Thursdays.
The House agreed to the amendment, and President Roosevelt signed the resolution on December 26, 1941, thus establishing the fourth Thursday in November as the Federal Thanksgiving Day holiday.
H.J. Res. 41, 10/6/1941, Records of the U.S. House of Representatives
Senate Amendments to H.J. Res. 41, 12/9/1941, Records of the U.S. Senate