<P> The mat is usually replaced twice per year: before Memorial Day and before Veterans Day . This is required because of the wear on the rubber mat by the special shoes worn by tomb guards . The sentinels have metal plates built into the soles and inner parts of their shoes to allow for a more rugged sole and to give the signature click of the heel during maneuvers . The sentinels wear sunglasses because of the bright reflection from the marble surrounding the Tomb and the Memorial Amphitheater . </P> <P> On the ground not covered by the mat, a rust pattern in the tile can be seen that corresponds to the precise steps taken during the changing of the guard . The metal from the guards' boots causes the brown rust markings on the stone . On the mat itself, footprints caused by standing guard are also visible . </P> <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>

When do they change guards at the tomb of the unknown soldier
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