Beginning as "Decoration Day" for the custom of tending to the gravestones of the war dead, a loosely-organized grassroots day of observance for the casualties of the recent bloody Civil War eventually became established on May 30th as Memorial Day. In 1968, Congress moved it to the last Monday in May where it now serves as the endcap of a weekend full of motor racing and barbecues and the beginning of summer.
"On the eleventh hour of the eleventh day of the eleventh month, the guns fell silent" and this was incorporated in the U.S. official calendar as Armistice Day, until Congress changed it to a day to remember all American veterans. Too late in the year for good picnicking, it's now a good time for sales at Macy's.
Similarly, Mike at Cold Fury asks how long it will be until people are wishing each other "Happy 9/11!" on the way to get a discount on mattresses or linens.
If history is any guide, probably another thirty or forty years.