I have done the anthropological research on this holiday for you. Voilà, my findings.
Thanksgiving was originally a holiday observed in the United States of America and Canada on Earth that enabled people to celebrate being thankful for what they had. It was still celebrated as of the 2370s.
…In 2266, the crew of the USS Enterprise were prepared to celebrate Thanksgiving. Captain Kirk was irritated because there was not any turkey on board the ship; the crew was going to have to eat meatloaf instead. Kirk was later informed that, while in the ovens, the meatloaf mysteriously transformed into turkey. This transformation was caused by Charles Evans, a boy with incredible powers. (TOS: “Charlie X“)
In 2372, Benjamin Sisko invited the entire senior staff of Deep Space 9 to Thanksgiving dinner. The captain had been growing vegetables for months to serve at the dinner. A year later, Michael Eddington told Sisko that he thought his former commanding officer had added too much tarragon to the stuffing. (DS9: “Blaze of Glory“)
So my takeaway from studying the above passages about this regional Earth tradition is that it had something to do with a minor Divinity miraculously transforming meatloaf into turkey to feed the hungry.
While the source is not totally clear, meatloaf is made from ground meat, presumably of any type. If this was turkey meat, then this would have been a turkey ground down and reconstructed as a loaf of meat bread, then subsequently re-deconstructed and re-reconstructed into a turkey form.
Thanksgiving therefore is essentially an Earth holiday about the endless cycle of death and rebirth. Destruction and creation. See also: Lord Shiva.