November 11th, 2008, will be the ninetieth anniversary of the end of World War I. Americans will celebrate this day as Veterans Day, and Canadians will celebrate it as Remembrance Day.
Fighting ended on November 11th, 1918 (though the agreement that officially ended the war was signed months later). Many countries made this a day to celebrate peace and honor those who had served in the war -- especially those who had died. It was called Armistice Day ("armistice" means an agreement to stop fighting).
Although World War I was called the war to end all wars, history did not work out that way. More wars were fought, and countries decided to use November 11th to honor those who had served in all of them. In 1931, Canada changed Armistice Day to Remembrance Day. In 1954, the U.S. changed Armistice Day to Veterans Day. (A veteran is someone who has served in the military.)
For a few years in the 1970s, the U.S. celebrated Veterans Day on a Monday, so people could have a long weekend. But this was not popular -- November 11th was too important to Americans. So the holiday was changed back.