Also called Decoration Day. a day, May 30, set aside in most states of the U.S. for observances in memory of dead members of the armed forces of all wars: now officially observed on the last Monday in May.
any of several days, as April 26, May 10, or June 3, similarly observed in various Southern states.
Origin of Memorial Day
An Americanism dating back to 1865–70
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a holiday in the United States, May 30th in most states, commemorating the servicemen killed in all American wars
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
used in a general sense is from 1830s; as a specific holiday commemorating U.S. war dead (originally Northern soldiers killed in the Civil War) it began informally in late 1860s. officially from 1869 among veterans' organizations.
Online Etymology Dictionary, © 2010 Douglas Harper