It was George Washington who declared the first federally recognized Thanksgiving Day, in 1789. I found this interesting tidbit on PBS's website:
"Thus begins the great American festival, known as Thanksgiving. It is alike a holy day, and a patriotic day. The genius of the American Founding is here demonstrated: Only in America could all citizens, of all religious persuasions, celebrate equally a day that is holy and patriotic. Only in a regime of religious liberty could members of different religions unite together as equal citizens. Such a thing would have been unimaginable, for example, in 17th or 18th century Europe. George Washington understood well the significance of this achievement. By enshrining it in a national holiday, Washington continues to remind us today of the source of all that we enjoy as American citizens."
Well said. Happy Thanksgiving.