We commonly refer to the first Thanksgiving as an event between the Pilgrims and Indians back in the 1600's.  The celebration took place after their first harvest in the New World in 1621 and lasted three days.  One attendee noted that there were 90 Native Americans and 53 Pilgrims at the celebration. 

In 1789, Congress requested a proclamation by George Washington, but Thomas Jefferson chose not to observe the holiday, so the celebration was intermittent.  In 1863, during Abraham Lincoln's presidency, Thanksgiving became a federal holiday.  Lincoln proclaimed this day a national day of "Thanksgiving and Praise to our beneficent Father who dwelleth in the Heavens".  He proclaimed it to be celebrated on the fourth Thursday in November.

In 1939, November had five Thursdays.  President Franklin D. Roosevelt declared the fourth Thursday as Thanksgiving rather than the fifth one.  His plan was to establish the holiday on the next to last Thursday in the month instead of the last one. He thought an earlier Thanksgiving would give merchants longer period to sell goods before Christmas with a chance to increase profits and spending during this period.  Roosevelt's thought was that this might help bring the country out of the Depression.  Within two years, the change passed through Congress into law.

For more Thanksgiving facts and information, and to view the sources of this blog, click on the link below:

https://en.wikipedia.org/wiki/Thanksgiving_(United_States)#Post-Civil_War_era