Every Memorial Day, thousands of flags are placed in front of the graves by the soldiers from the 3rd U.S. Infantry Unit (also known as Old Guard) during a three hour ceremony called a "Flags-In"
Arlington National Cemetery is spread over 624 arces and it has the second-largest number of people buried at any of the 130 national cemeteries in the US. For more info on Arlington National Cemetery's rich history, check out its web site at http://www.arlingtoncemetery.org/