<P> While Arlington National Cemetery is open, during the day in summer months from April 1 to September 30, the guard is changed every half hour . During the winter months, from October 1 to March 31, the guard is changed every hour . After the cemetery closes to the public (7 p.m. to 8 a.m. April through September, and 5 p.m. to 8 a.m. October through March), the guard is changed every 2 hours . The ceremony can be witnessed by the public whenever Arlington National Cemetery is open . </P> <P> The guard change is very symbolic, but also conducted in accordance with Army regulations . The relief commander or assistant relief commander, along with the oncoming guard, are both required for a guard change to take place . The relief commander orders the guard being relieved to "pass on your orders" to the oncoming guard . The guard being relieved will say to the oncoming guard, "Post and orders remain as directed ." The oncoming guard's response is always, "Orders acknowledged ." During changes when the public is witnessing the ceremony, the commander will inform the public that the ceremony is about to take place and that those in attendance should remain "silent and standing" throughout the entire event . </P> <P> A civilian guard was first posted at the Tomb on November 17, 1925, to prevent, among other things, families from picnicking on the flat marble slab with views of the city . A military guard was first posted on March 25, 1926 . The first 24 - hour guard was posted on midnight, July 2, 1937 . The Tomb of the Unknowns has been guarded continuously, 24 hours a day, 7 days a week, since that time . Inclement weather, terrorist attacks, et cetera, do not cause the watch to cease . </P> <P> Since 1948, the tomb guards, a special platoon within the 3rd U.S. Infantry Regiment (The Old Guard) work on a team rotation of 24 hours on, 24 hours off, for five days, taking the following four days off . A guard takes an average of six hours to prepare his uniform--heavy wool, regardless of the time of year--for the next day's work . In addition to preparing the uniform, guards also conduct physical training, tomb guard training, participate in field exercises, cut their hair before the next work day, and at times are involved in regimental functions as well . Tomb guards are required to memorize 35 pages of information about Arlington National Cemetery and the Tomb of the Unknown Soldier, including the locations of nearly 300 graves and who is buried in each one . </P>

When is the tomb of the unknown soldier guarded