The Changing of the Guard takes place daily at 11.30 a.m. from May to July, in front of the the Buckingham Palace's gates. The rest of the year it takes place every two days, excepting raining days when the performance is cancelled.
Different and Exciting, I saw this as a child--a very interesting tradition. If you have time go, its fun as long as it's not sweltering or raining!
