For so many people, this is a national day of shopping. For Native Americans, the day after Thanksgiving is a time of betrayal and genocide. Here, and Here, and there’s a lot of information about the original tribe, the Wampanoag and what they did for the settlers here
Since 1970, the day after Thanksgiving has become a Native American National Day of Mourning to “honor their ancestors and the struggles of Native people to survive today.” Source
There is a ceremony held in Plymouth Massachusetts, and Native and non-Native people alike go to participate in it.