ok so I dont know the first one, but sand creek massacre happend on 1864 Novermber 29th, basically a group called the Colorado Volunteers surrounded the Natives at Sand creek, massacring them all, killed about 400 people, most scalped.
Massacre at wounded knee: happend on the Lakota Pine Ridge Indian Reservation, December 29th, 1890. The 7th Calvary Regiment surrounded the Indians once more massacring 150-300 Lakots were dead. 25 soilders died, and 39 wounded.