Today is Veterans Day, originally designated to celebrate the "Armistice" and to honor those soldiers who served in "the Great War." The First World War ended on "the 11th hour of the 11th day of the 11th month" in 1918. A Congressional resolution called for Armistice Day to became an official US holiday IN 1926. An act of Congress made it official in 1938. In 1954 the holiday was changed to Veterans Day, honoring all veterans, not just WWI vets.
From 1971-1977, Veterans Day was moved to the 4th Monday in October, but returned to Nov. 11 in 1978.
Personally, I have ancestors who served in the Revolutionary War, War of 1812, Civil War, the War in the Philippines, WWII and during peacetime.
If you are interested in researching your military ancestors, there are several websites that can be of help. Fold3.com is dedicated to military research and is an excellent place to begin. Ancestry.com, WorldVitalRecords.com and FamilySearch.org all have military records available. Check local archives for state military databases. The Civil War Soldiers and Sailors System is an excellent site for finding information on Union and Confederate veterans. The National Archives [NARA.gov] also has military records.
One very important source to consult is the pension file. There is potentially a ton of information to be gleaned from the veterans' and widows' pensions.