This happens every hour on the hour (every 30 minutes in summer and winter, due to weather extremes) when the guards change, standing watch at the Tomb of the Unknown Soldiers at Arlington National Cemetery in Washington DC. Each guard change, visitors can watch the NCO come out, explain what they are about to watch, inspect the oncoming guard, stand by as the guards change and render a salute to the unknown soldiers. It is a moving ceremony to watch.
The uniforms the guards wear is only worn in cold weather.