The Collaborative International Dictionary of English v.0.48:

Memorial Day \Me*mo"ri*al Day\
   1. A day, originally May 30, appointed for commemorating, by
      decorating their graves with flowers, by patriotic
      exercises, etc., the dead soldiers and sailors who served
      the Civil War (1861-65) in the United States; Also called
      Decoration Day. It is a legal holiday in most of the
      States. In the Southern States, the Confederate Memorial
      Day is: May 30 in Virginia; April 26 in Alabama, Florida,
      Georgia, and Mississippi; May 10 in North Carolina and
      South Carolina; the second Friday in May in Tennessee;
      June 3 in Louisiana. [U. S.]
      [Webster 1913 Suppl.]

   2. A day designated for commemoration of all of the war dead
      of the United States, clebrated on the last Monday in May
      in most states. It supersedes the original Memorial Day
      celebrated May 30th.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Memorial \Me*mo"ri*al\, a. [F. m['e]morial, L. memorialis, fr.
   memoria. See Memory.]
   [1913 Webster]
   1. Serving to preserve remembrance; commemorative; as, a
      memorial building.
      [1913 Webster]

            There high in air, memorial of my name,
            Fix the smooth oar, and bid me live to fame. --Pope.
      [1913 Webster]

   2. Contained in memory; as, a memorial possession.
      [1913 Webster]

   3. Mnemonic; assisting the memory.
      [1913 Webster]

            This succession of Aspirate, Soft, and Hard, may be
            expressed by the memorial word ASH.   --Skeat.
      [1913 Webster]

   Memorial Day. See Memorial Day in the vocabulary. Also
      called Decoration Day. [U.S.]
      [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Decoration Day \Decoration Day\
   a day, May 30, originally appointed for decorating with
   flowers the graves of the Union soldiers and sailors, who
   fell in the Civil War in the United States; -- now called
   Memorial Day, and established as the last Monday in May,
   and designated as a day for commemorating those who died in
   all wars of the United States. [U. S.]
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

Memorial Day
    n 1: legal holiday in the United States, last Monday in May;
         commemorates the members of the United States armed forces
         who were killed in war [syn: Memorial Day, Decoration
         Day]