Answer:
In 1789, President George Washington became the first president to proclaim a Thanksgiving holiday, when, at the request of Congress, he proclaimed November 26, a Thursday, as a day of national thanksgiving for the U.S. Constitution. Thanksgiving was to expressing gratitude for a pivotal Union Army victory at Gettysburg.
Supporter for Thanksgiving national Holiday:
Sarah Josepha Hale
Sarah Josepha Hale, a 74-year-old magazine editor, wrote a letter to Lincoln on September 28, 1863, urging him to have the "day of our annual Thanksgiving made a National and fixed Union Festival."
Have a great day! :D