The following is a quote from Igros Moshe, authored by Rav Moshe Feinstein zt”l.
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Explanation:
Rav Moshe was entertaining the possibility that it would even be forbidden to make a LEGITIMATE Simcha, like a Bar-Mitzvah or Chasunah celebration on Thanksgiving, since it would APPEAR that one is celebrating Thanksgiving!
To THAT Rav Moshe writes that it isn’t forbidden according to the letter of the law, but still a Ba’al Nefesh shouldn’t do so!
Then Rav Moshe continues and writes that celebrating or making a Seudah SPECIFICALLY for Thanksgiving is CERTAINLY FORBIDDEN. (???? ?? ????? ?????)
To summarize:
One is forbidden to celebrate Thanksgiving or make a special Seudah in its honor!
However, to schedule a legitimate Simcha, like a Bar-Mitzvah or Chasunah, on Thanksgiving, is Halachikly permitted but a Ba’al Nefesh shouldn’t do even that, and should rather reschedule his legitimate Simcha for some other day!
Accordingly, it would be forbidden to eat turkey on Thanksgiving.
Because if it is being eaten as a full meal; it is clearly forbidden, since it falls under the category of making a Seudah.