The United States will play a friendly against Peru on September 4, according to the Washington Post. The match will be played at RFK Stadium in Washington, D.C., bringing the Americans back to one of the most historic venues in the country's soccer history.
September 4 falls within a FIFA international window so Jurgen Klinsmann will be able to call in whichever players he would like and clubs will have to release them. There will be some conflict with MLS matches being played that weekend, but because the friendly is being played in the U.S. and travel won't be a significant problem, Klinsmann shouldn't have too much opposition to calling in the players from the league that he would like.
The FIFA international date extends into the next week so the U.S. will probably schedule another friendly on the 8th or 9th, also on home soil.
Peru are in the semifinals of the ongoing Copa America. They're a surprise team to make it this far and you'd be hard-pressed to find someone who believes they are one of the four best teams in South America, but they are a good side with the likes of Jefferson Farfan and Paolo Guerrero.