In front of the Tomb of the Unknown Soldier. September 18, 2014
The Tomb of the Unknown Soldier is guarded 24 hours a day, every day of the year, by Tomb Guard sentinels. Sentinels are all volunteers from the 3rd U.S. Infantry Regiment (The Old Guard), headquartered at Fort Myer, Va. The "Old Guard" dates from 1784 and is the oldest active duty infantry unit in the Army.

http://www.arlingtoncemetery.mil/Explore-the-Cemetery/Changing-of-the-Guard