In a stirring tribute to mark Memorial Day each year, all available soldiers of the elite 3rd U.S. Infantry (known as The Old Guard) gather at Arlington National Cemetery to perform a special task. Just before the Memorial Day weekend, they place American flags, one foot and centered, in front of the gravestones and columbarium niches of every service member buried or inurned at Arlington Cemetery.
This tradition of honor, known as Flags-in, has taken place ever since 1948 when The Old Guard was appointed as the ceremonial unit for the U.S. Army. During the Memorial Day weekend, members of The Old Guard patrol the cemetery to make sure each gravesite remains decorated and honored with a flag. In addition, sentinels for the Tomb of the Unknowns place flags at each of the unknown servicemen graves.
The flags are removed after the three-day Memorial Day weekend.
(I have to give credit for this information from Sheridan Alexander from About.com)