ARLINGTON, Va. -- About 1,000 soldiers with the 3rd U.S. Infantry Regiment, known as the "Old Guard," honored America's departed heroes yesterday by observing a longstanding Memorial Day weekend tradition known as the 'Flags In.' The Soldiers placed American flags at the gravesites of service members buried at Arlington National Cemetery and at the U.S. Soldiers' and Airmen's Home National Cemetery in Washington, D.C.TRADITIONEstablished in 1784, the 3rd Infantry Regiment is the oldest active-duty infantry unit in the Army. The Old Guard maintains a 24-hour vigil at the Tomb of the Unknown Soldier and provides military funeral escorts at Arlington, among other duties."Flags-In" has been conducted annually since the Old Guard was designated as the Army's official ceremonial unit in 1948. Every available soldier in the 3rd U.S. Infantry Regiment participates.On Thursday, the Old Guard soldiers fanned out across Arlington's hills and valleys with rucksacks full of flags. They approached each headstone, centering a miniature flag exactly one boot length from the base before sinking it into the ground. In total, they placed the flags in front of more than 280,000 headstones and at the bottom of about 7,000 niche rows in the cemetery's Columbarium Courts and the Niche Wall.Tomb Sentinels also placed flags at the Tomb of the Unknown Soldier, and set approximately 14,000 flags at the Soldiers' and Airmen's Home National Cemetery in Washington, D.C. The flags will be removed after Memorial Day, before the cemeteries open to the public.HONORING DEPARTED SERVICE MEMBERSArmy Pvt. Gabriel Thyfault, a truck driver with the Old Guard, said that he and the other soldiers read the names on the headstones as they placed flags on the graves."It's a huge honor. I've never experienced anything like this," said Thyfault, who hails from Chicago. "I couldn't be more thankful to be out here, putting a flag on every single grave in the entire cemetery. It's such an overwhelming honor."Thyfault said his father served in the Navy, and his uncle served in the Air Force, both during Desert Storm.Army Staff Sgt. David Rivera, a squad leader from Orlando, Fla., also shared the special honor he felt in participating in this hallowed tradition. The day was meaningful for him, Rivera said, because he was able to place flags on the graves of friends who paid the ultimate sacrifice when he was deployed with them in Iraq in 2010.For Army Pvt. Wes DeFee, an Old Guard medic from Charleston, S.C., the Flag In offers a tangible opportunity to reflect on the meaning of Memorial Day. "All these people out here have given their lives for our daily freedom to live in this great country the way we do. It shouldn't be taken for granted."Thyfault agreed, adding, "Take time to honor those who have paid the ultimate sacrifice, the ones who have served and given their lives for their country."