In 1970, for the 350th anniversary of the Pilgrims' landing, Wamsutta Frank James, an Aquinnah Wampanoag man, was invited to speak. When the organisers read his speech in advance, he was not allowed to deliver it as written. He spoke instead on Cole's Hill in Plymouth on Thanksgiving Day, and the first National Day of Mourning was held. It has taken place every year since.
Read the source: Pilgrim Hall Museum, National Day of Mourning · United American Indians of New England, the speech