The Department of Veteran's Affairs has a useful history of Veteran's Day:
"World War I –- known at the time as 'The Great War' -- officially ended when the Treaty of Versailles was signed on June 28, 1919, in the Palace of Versailles outside the town of Versailles, France. However, fighting ceased seven months earlier when an armistice, or temporary cessation of hostilities, between the Allied nations and Germany went into effect on the eleventh hour of the eleventh day of the eleventh month. For that reason, November 11, 1918, is generally regarded as the end of 'the war to end all wars.'"
"In November 1919, President Wilson proclaimed November 11 as the first commemoration of Armistice Day with the following words: 'To us in America, the reflections of Armistice Day will be filled with solemn pride in the heroism of those who died in the country’s service and with gratitude for the victory, both because of the thing from which it has freed us and because of the opportunity it has given America to show her sympathy with peace and justice in the councils of the nations…'"