While it's a nice idea to believe that America's Thanksgiving Day is rooted in the Bible, the fact is, it is not. Our Thanksgiving Day in America has nothing whatsoever to do with what YHWH told Israel thousands of years ago. Thanksgiving Day in America started solely because our country was founded "under God" and our leaders decided to turn it into a national holiday.
Leviticus 3:17 and Lev 7:11-15 are all about PEACE offerings that have nothing to do with our national "Thanksgiving Day" in America. These passages weren't referring to any kind of holy day; they were discussing peace offerings to be done by the cohanim for YHWH's people. America, as a whole, does not follow any of the Biblical Feasts nor bother with offerings. Americans, as a whole, don't follow Torah at all; they're into "Christmas and Easter" and man-made ideas that have nothing whatsoever to do with YHWH.
Thanksgiving Day in America is supposed to be a day of thanksgiving to God - but, judging from what we see on television, it's all about commercialism and eating turkey and ham and stuffing ourselves, and many don't bother thinking of God at all on Thanksgiving as they refer to it as "Turkey Day." Thanksgiving Day has nothing whatsoever to do with the Levitical laws given in Israel thousands of years ago.
If Thanksgiving Daywere rooted in the Bible, we would have yet another "pagan" holiday on our hands because today, Thanksgiving dinner on many tables includes "unclean foods" such as ham and white gravy with bacon bits, and Jell-O and all sorts of other things YHWH does not approve of.