Changing of the Guard
Changing of the Guard ceremonies are popular with tourists and other observers because of the formal precision with which they are carried out, making them interesting to watch, and also because the thing being guarded is normally one of great significance, making the ceremony highly evocative as well.
The guards involved are often highly trained units; the guarding of the particular place may be their only official duty. In some instances, such as the changing of the guard ceremony on Parliament Hill in Ottawa, the guards are primarily reservists receiving military training during the summer while attending university. 
The world's best known Changing of the Guard is probably the Changing of the Queen's Guard at Buckingham Palace, London and other royal residences. The Changing of the Guard at the Tomb of the Unknowns, Arlington National Cemetery, in the U.S. state of Virginia, is carried out every 20 minutes by a special military unit and also attracts thousands of visitors per year.
-  Schedule and information from the Canadian Parliament's website.