The first "official" Thanksgiving did not occur with Pilgrims. The first "official" Thanksgiving Holiday was in 1863. Signed into law by President Lincoln during a time of great turmoil and strife in our nation. The Civil War was at it's height, the Battle of Gettysburg had occurred that summer with over 60,000 Americans dying over a three day period, riots had erupted in New York City over Lincoln's Draft law and the political pundits of the day had declared him a one-term president who had tried to destroy the nation.
And in these troubled times, Lincoln asked the country to give thanks!