Decoration Day was the predecessor to the American holiday Memorial Day. (Though I have found one site picturing a Decoration Day parade in Chatham, Ontario.) Some people continued using the old name or both names. According to some sources, it was suggested by Henry C. Welles of Waterloo, New York, a few months after the end of the American Civil War in 1865 in to commemorate those killed in action, but was not officially observed until 1868 at the instigation of General John A. Logan. However, others credit it to the women of Columbus, Georgia, or Columbus, Mississippi, who in 1866 or 1867 are said to have decorated both Northern and Southern graves in their area. There are several other locations of origin mentioned in different places, but those are the most frequently given. No matter who started it, it spread to all areas affected by the war.

Traditionally it's May 30, but some areas seem to have had grave decorations (and not just those of soldiers) on July 4 or other summer dates also. The 1892 story "Decoration Day" by Sarah Orne Jewett says it's celebrated on that date whatever day of the week it fell on, unlike our modern three day weekend holidays). In that story it is celebrated with a parade of the still-living Civil War veterans, and "No matter if every other day in the year they counted for little or much, whether they were lame-footed and despised, whether their farms were of poor soil or rich," on this day they were respected and honored. Flags and flowers on the graves of those who died in that war, a child playing "Taps" on a fife for them, and a speech by a minister. These seem to be fairly typical observances.

The name change to Memorial Day seems to have been gradual, with "Memorial Day" being used as early as 1884 by Oliver Wendell Holmes but not really catching on until after World War I, and the change to the last Monday of May was done by Congress in 1971.

It's also an EP by a band called Town & Country, an album by Drive-By Truckers, and a 1990 TV movie starring James Garner about a friend of his character's turning down the Congressional Medal of Honor. And last, but not least, it's the name of a blues song, written and performed by Howlin' Wolf (Chester Burnett) and also recorded by Sonny Boy Williamson which reinforces the idea of a holiday to remember everyone and not just soldiers:

Lord, I've got a woman, she's nice lovin' in every way
Lord, I've got a woman, she's nice lovin' in every way
You know she done died and left me; I had a blues on every decoration day

So sorry to see you to leave me, I hate to see my baby get away
So sorry to see you leave me, I hate to see my baby get away
I want ya to bring me some flowers, about every decoration day

Lord, I was walkin' 'round her bedside, these was the last words my baby had to say
Lord, walkin' 'round the bedside, these was the last words my baby had to say
She said, "Bring me some flowers, be sure, honey on every decoration day"

Sources:
http://oldfashionedholidays.com/decorationday.shtml
http://etext.lib.virginia.edu/toc/modeng/public/JewDeco.html
http://www.bartleby.com/65/me/MemorialD.html
http://www.worldwideschool.org/library/books/lit/
literarystudies/PonkapogPapers/chap8.html
http://www.br28legionpipeband.com/decday98.html
http://www.domicilemagazine.com/issues/200007/decoration.html
http://blueslyrics.tripod.com/lyrics/howlin_wolf/decoration_day.htm