The fourth Thursday in November marks Thanksgiving Day for most Americans. But to some, it’s the “National Day of Mourning.”

For more than half a century, groups of Native Americans have been gathering on Thanksgiving to mark a National Day of Mourning at a historic site in Massachusetts. They honor their ancestors who were subject to atrocities by settlers in the 17th century.

The ceremony dates back to 1970 when Kisha James‘ grandfather was invited to speak at a commemoration of the 350th anniversary of the Pilgrims’ arrival in Plymouth.