When I came to the US, as college student knowing nothing about American culture, I was faced with this holiday in the Fall that I did not understand. It's not like I was a little kid and there would be pageants and Indians and Pilgrims and someone to explain it all to me. So in my head I translated "Thanksgiving" to "The act of giving 'thanks'." And I remembered that there was a religious holiday in Brazil called "Ação de Graças", meaning pretty much the same thing, also in November. So I went through the first couple of Thanksgivings in the US believing that Thanksgiving was a religious holiday. I spent the first two with American families, who also did not find any reason to explain the basics to me, because they assumed I knew.
Not until my 3rd year in college, when I was interviewed by a local newspaper (they asked the college for a couple of foreign students to interview for an article), did I realize that it wasn't a religious holiday after all. I felt silly, because it was the journalist who explained it to me. Of course, that ended up in the paper, which ended up on the coffee table in the house of my academic advisor, which is where I spent that Thanksgiving. I felt both famous and silly.
And only recently did I look this up again, and the truth is that it is one and the same holiday. It originated in the US, and Brazil's ambassador to the US in 1949 liked the thought of everyone around the world taking a day to be thankful, and suggested it become a holiday in Brazil too. However, when it was brought to Brazil, a predominantly Catholic country, it took on a religious meaning, and most of the thanks given on the Brazilian Thanksgiving is to God. And it is not a day that families get together, and it is not a day off from work and school. So that explains my ignorance.
Regardless, I agree with the ambassador who thought it is a wonderful time to reflect and be thankful for everyone and everything in one's life. I am surrounded by people that enrich my life, and my work is fulfilling and worthwhile. I am very thankful for that, and much more.