At the Tower of London, the Ceremony of the Keys has been going on every night in some form or another since the 14th century. At exactly 9:53pm, the Chief Yeoman Warder meets the military escort and they secure the main gates of the tower.
When they return down Water Lane, the party is halted by the sentry and is challenged to identify themselves. After, they go to the Broadwalk steps where they make a salute to the Queen and then the Warder takes the keys for safe keeping.
The only notable hiccup in this ceremony since the 14th century was one time during World War II. The bombings over London delayed the ceremony. The Officer of the Guard sent a letter to King George apologizing for the delay. The king replied that he should not be punished because the delay was a result of enemy action.