Why was Thanksgiving in the US traditionally the last Thursday before Advent? Prior to a change instituted by President Roosevelt I understand United States Thanksgiving was the last Thursday in November.

This is invariably the last Thursday before Advent Sunday (Nov 27 - Dec 3).

My question is whether this was deliberately intended to be the last Thursday before Advent, or if not was there some other reason.
Why was Thanksgiving in the US traditionally the last Thursday before Advent?

The Short answer is: There seems to be no connection between Thanksgiving Day and the First Sunday of Advent. However, the number of shopping days after Thanksgiving influenced the move from the Last Thursday of November (1863) to the Fourth Thursday of November (1939).

In fact the First Thanksgiving was held by the Pilgrims in October 1621.


  The event that Americans commonly call the "First Thanksgiving" was celebrated by the Pilgrims after their first harvest in the New World in October 1621. This feast lasted three days, and—as accounted by attendee Edward Winslow — it was attended by 90 Native Americans and 53 Pilgrims. The New England colonists were accustomed to regularly celebrating "thanksgivings"—days of prayer thanking God for blessings such as military victory or the end of a drought. - Thanksgiving (United States)


While turkey is today the bird of choice for Thanksgiving dinners across the United States, this was not always the case: according to History.com, for the first ever Thanksgiving in 1621 the Indians killed five deer as a gift for the colonists, meaning venison would most likely have been the dish of the day.


  Thanksgiving is a national holiday in the United States, celebrated on the fourth Thursday of November. It originated as a harvest festival. Thanksgiving has been celebrated nationally on and off since 1789, with a proclamation by George Washington after a request by Congress. Thomas Jefferson chose not to observe the holiday, and its celebration was intermittent until the presidency of Abraham Lincoln, when Thanksgiving became a federal holiday in 1863, during the American Civil War. Lincoln proclaimed a national day of "Thanksgiving and Praise to our beneficent Father who dwelleth in the Heavens," to be celebrated on the last Thursday in November. Under President Franklin D. Roosevelt, the date was changed to the fourth Thursday in November, an innovation that endures to this day. Together with Christmas and the New Year, Thanksgiving is a part of the broader fall–winter holiday season in the U.S. Thanksgivng (United States) 


President Abraham Lincoln chose that the Last Thursday of November to be that of Thanksgiving because of the decisive victory the Union Army had at Gettysburg.  


  On this day (October 3) in 1863, expressing gratitude for a pivotal Union Army victory at Gettysburg, President Abraham Lincoln announces that the nation will celebrate an official Thanksgiving holiday on November 26, 1863. - Lincoln proclaims official Thanksgiving holiday


The final change from the Last Thursday of November to the Fourth Thursday of November was not influenced by the First Sunday of Advent but by the number of shopping days left before Christmas.

At the beginning of Franklin Roosevelt's presidency, Thanksgiving was not a fixed holiday; it was up to the President to issue a Thanksgiving Proclamation to announce what date the holiday would fall on. However, Thanksgiving was always the last Thursday in November because that was the day President Abraham Lincoln observed the holiday when he declared Thanksgiving a national holiday in 1863. Franklin Roosevelt continued that tradition, but he soon found that tradition was difficult to keep in extreme circumstances such as the Great Depression. His first Thanksgiving in office, 1933, fell on November 30th, the last day of the month, because November had five Thursdays that year. Since statistics showed that most people did not do their Christmas shopping until after Thanksgiving, business leaders feared they would lose money, especially during the Depression, because there were only 24 shopping days between Thanksgiving and Christmas. They asked Franklin Roosevelt to make Thanksgiving one week earlier. President Roosevelt ignored those concerns in 1933, but when Thanksgiving once again threatened to fall on the last day of November in 1939, FDR reconsidered the request and moved the date of Thanksgiving up one week. Thanksgiving 1939 would be held, President Roosevelt proclaimed, on November 23rd and not November 30th. 


  The fourth Thursday of November remained the annual day of Thanksgiving from 1863 until 1939. Then, at the tail-end of the Depression, President Franklin D. Roosevelt, hoping to boost the economy by providing shoppers and merchants a few extra days to conduct business between the Thanksgiving and Christmas holidays, moved Thanksgiving to November’s third Thursday. In 1941, however, Roosevelt bowed to Congress’ insistence that the fourth Thursday of November be re-set permanently, without alteration, as the official Thanksgiving holiday. - Lincoln proclaims official Thanksgiving holiday