Seems to be on or about 20 May, 2010;
From media release found at various sources:
“Chad Smith, chief of the Cherokee Nation, said most tribes had not specifically asked for a formal apology from the US government, but the gesture was appreciated.
“It’s difficult to issue an apology and sometimes it’s difficult to accept one,” Mr Smith said.
“Once you put those differences of the past aside, perhaps the next step is, can you do any better in this round?
That is where our greatest challenge is. The history of the US (toward American Indians) is not a bright record. The real question is, what happens from this day forward? (emphasis added)”