The History Channel webpage features the day's history:
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...The Library of Congress features the History of Memorial Day.
There is more in a helpful round-up on the American Historical Association blog.
The Law Librarians Blog features two Memorial Day Addresses by Oliver Wendell Holmes, Jr.
Image: Decoration Day.