From Academic Kids
Armistice Day is the anniversary of the official end of World War I, November 11, 1918. It commemorates the armistice signed between the Allies and Germany at Compiègne, France, for the cessation of hostilities on the Western Front, which took effect at eleven o'clock in the morning — the "eleventh hour of the eleventh day of the eleventh month." While this official date to mark the end of the war reflects the ceasefire on the Western Front, hostilities continued in other regions, especially across the former Russian Empire and in parts of the old Ottoman Empire.
This was celebrated as a national holiday each year in many of the former allied nations. After World War II, it was changed to Veterans Day in the United States and to Remembrance Day in the Commonwealth of Nations. In many parts of the world, people take a minute of silence at 11:00 as a sign of respect.
By extension, many people observe a similar minute (or more) of silence in respect of other deathful events (the most prominent examples being the September 11, 2001 attacks and the 2004 Indian Ocean earthquake). More information on this can be found in the article on silence.