Oh, that woman has probably read Zinn's Marxist People's History of America (maybe the title is a little different, but if you Google Zinn you'll find the right one). I have read it. It is interesting, but very polemical. Basically, European settlement of the US was a terrible, bad thing, and shouldn't have happened. Those of us who are descendants of those European explorers and settlers should live in eternal shame and adopt a permanent stance of apology. According to these Marxist historians and their followers, no white European ever did anything worthwhile in the history of the world. I don't recall how Zinn handles the human sacrifice that was going on among some American aborigines (for example, among those who built the pyramids outside Mexico City) before the European "invasion," as he terms it. I guess he thinks whatever they were doing was A-OK, because it was an authentic expression of their native culture, etc.
As for Thanksgiving, which this principal also impugns, that was simply a harvest festival, which was common among the Puritans even before they got here. I'm not in a position to know how many Indians took part in it, but apparently there were a few present. I guess she is steamed that we have a national holiday which acknowledges some type of deity (your choice) over the world.
Not sure what her beef is about Halloween--the article doesn't explain that. Usually it is the evangelicals who object to that "pagan" celebration, but I'm guessing that, in this woman's case, it is just too indicative of our "European perspective."