Thought I would share this from a couple of years ago. The changing of the Guard occurs every hour during the Winter and every 30 minutes during the Summer. The Tomb Guard marches exactly 21 steps down the black mat of the Tomb, turns, faces east for 21 seconds, turns and faces north for 21 seconds, then takes 21 steps down the mat and repeats the process. Note that the sound on the video is not muted. It is done in complete silence.