How was the date on which Americans remember their war dead arrived at ? In Europe we chose the anniversary; and the very hour of the Armistice in WW1 for the two minute silence: The eleventh hour of the eleventh day of the eleventh month.

Here's some info I found on the subject:

The selection of May 30th is attributed to a Virginian of French descent, Cassandra Oliver Moncure, who may have selected this date because it was "The Day of Ashes" in France-the day that Napoleon's remains were returned to France from St. Helena.

Memorial Day's origins date back to the Civil War. One story has four women scattering flowers on the graves of Union and Confederate graves where, four years earlier, the battle of Shiloh took place. Miziuk notes though, that they didn't start this tradition until April 26, 1866, almost two years after Miss Hunter began her vigil.
Another has Emma Hunter of Boalsburg, Penn. carrying flowers to her father's grave. Her father was Col. James Hunter, commander the 49th Pennsylvania Regiment during the Civil War. Col. Hunter was killed at the battle of Gettysburg, and Miss Hunter carried flowers to his grave at Gettysburg Cemetery in 1864. Soon, the local townspeople started doing the same for all soldiers.
"The Grand Army of the Republic (GAR), a veterans' organization, was impressed with the concept of honoring fallen soldiers. The GAR organized a Decoration Day service at Arlington National Cemetery on May 30, 1868.
Eventually, New York newspapers heard of the event, and people all along the coast followed suit. (New York was the first state to adopt "Decoration Day" later called Memorial Day as a legal holiday, doing so in 1873. In time, with other wars, what was a day to honor the CIvil War dead became a day to honor all those who died in the service of their country. The alternative name of "Memorial Day" was first used in the 1882, but did not become more common until after World War II, and was not declared the official name by Federal law until 1967. In 1971, President Richard Nixon declared Memorial Day a national holiday to be held on the last Monday in May.
November 11 in the US is Veterans Day (formerly Armistice Day). The distinction is that Memorial Day honors the U.S. soldiers who died in the wars, while Veterans Day honors those who survived.

Below from The History Channel

Memorial Day was originally known as Decoration Day because it was a time set aside to honor the nation's Civil War dead by decorating their graves. It was first widely observed on May 30, 1868, to commemorate the sacrifices of Civil War soldiers, by proclamation of General John A. Logan of the Grand Army of the Republic, an organization of former sailors and soldiers. On May 5, 1868, Logan declared in General Order No. 11 that:

The 30th of May, 1868, is designated for the purpose of strewing with flowers, or otherwise decorating the graves of comrades who died in defense of their country during the late rebellion, and whose bodies now lie in almost every city, village, and hamlet churchyard in the land. In this observance no form of ceremony is prescribed, but posts and comrades will in their own way arrange such fitting services and testimonials of respect as circumstances may permit.

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Originally posted by FredPuli:
How was the date on which Americans remember their war dead arrived at ? In Europe we chose the anniversary; and the very hour of the Armistice in WW1 for the two minute silence: The eleventh hour of the eleventh day of the eleventh month.

When I was a kid, we called that "Armistice Day" and celebrated it as such. After another couple of wars, the "War to end war" lost some of it's appeal, so we still celebrate it, but call it Veterans Day instead.

Alan Moore