From pop culture and my idiot grade school social studies teachers, of course.
Thinking back, my history teachers were some of the worst teachers I had, once I got to middle school and high school. No mastery of their material and little ability to get the class interested. At least my civics/government teacher was a football coach who could command the attention of the classroom.
Maybe I learned about Thanksgiving originally there then. But all I can recall about that is drawing one using my hand in art class.