How did Thanksgiving become a national holiday in the united States?

It is on a Thursday in November in a year a long time ago.

This question has a lot of history behind it - allow me to tersely recapitulate such: On 11/23/39, FDR carved the the turkey at the annual Thanksgiving dinner in Georgia. However, up until this date, most Americans had celebrated Thanksgiving on 11/30.

During FDR's administration, Thanksgiving was not a fixed Holiday - it was up to the present President to declare what date Thanksgiving would be held that year in November.

When Lincoln had been President, he observed Thanksgiving on the last Thursday in November and declared it a national holiday in 1863.

However, during the Great depression, FDR found it difficult to observe Thanksgiving on the last Thursday of November.

During the great depression, FDR was approached to make Thanksgiving held on the 4th Thursday of November to allow more time for people to shop for Christmas. In 1939, FDR moved the Holiday one week up.