I think it is either the Feast of Passover, or Feast of Weeks. But, I'm pretty sure it's probably the Feast of Passover. Hope this helps you!
The exchange that most affected the indigenous people was the one of diseases. Native Americans hadn't encounter the diseases the European brought before, so they hadn't develop immunity to them. This is the major reason of the large death of Native Americans, besides the war between them and the brutality of the Europeans.