What city was the birthplace of the Memorial Day tradition in the United States?

In 1966, Congress and President Lyndon Johnson declared Waterloo, New York as the "birthplace" of Memorial Day.

However on May 5, 1868 General John Logan, national commander of the Grand Army of the Republic, proclaimed Decoration Day (the original name of Memorial Day.) It was first observed on May 30, 1868 at Arlington National Cemetery, Arlington County, Virginia when the graves of Union and Confederate soldiers were decorated with flowers.