What is now called Memorial Day in the United States began in the 1860s as “Decoration Day”—decorating the graves of dead soldiers with flowers. The first official “Decoration Day” was announced by General John A. Logan of the Grand Army of the Republic and held on May 30, 1868. The official announcement, on May 5, 1868, did not use the term “Decoration Day,” but explained:
 
“The 30th day of May, 1868, is designated for the purpose of strewing with flowers or otherwise decorating the graves of comrades who died in defense of their country during the late rebellion, and whose bodies now lie in almost every city, village, and hamlet church-yard in the land. In this observance no form of ceremony is prescribed, but posts and comrades will in their own way arrange such fitting services and testimonials of respect as circumstances may permit.”
 
The term “Decoration Day” has been cited in print since at least June 1867 (referring to Richmond, VA) and April 1868 (referring to Mississippi).
 
   
Wikipedia: Decoration Day
Decoration Day is the former name of Memorial Day in the United States. Memorial Day began as “Decoration Day” in the immediate aftermath of the American Civil War. It was a tradition initiated by former slaves to celebrate emancipation and commemorate those who died to end slavery in the United States.
 
History.com—This Day in History
May 30, 1868:
Civil War dead honored on Decoration Day
By proclamation of General John A. Logan of the Grand Army of the Republic, the first major Memorial Day observance is held to honor those who died “in defense of their country during the late rebellion.” Known to some as “Decoration Day,” mourners honored the Civil War dead by decorating their graves with flowers. On the first Decoration Day, General James Garfield made a speech at Arlington National Cemetery, after which 5,000 participants helped to decorate the graves of the more than 20,000 Union and Confederate soldiers buried in the cemetery.
 
The 1868 celebration was inspired by local observances that had taken place in various locations in the three years since the end of the Civil War. In fact, several cities claim to be the birthplace of Memorial Day, including Columbus, Mississippi; Macon, Georgia; Richmond, Virginia; Boalsburg, Pennsylvania; and Carbondale, Illinois.
 
John A. Logan College (Carterville, IL)
Logan’s Order Mandating Memorial Day
The following is the entire text of General Order No. 11, mandating the creation of the first Memorial Day, May 30, 1868:
Headquarters Grand Army of the Republic
General Orders No.11, WASHINGTON, D.C., May 5, 1868
I. The 30th day of May, 1868, is designated for the purpose of strewing with flowers or otherwise decorating the graves of comrades who died in defense of their country during the late rebellion, and whose bodies now lie in almost every city, village, and hamlet church-yard in the land. In this observance no form of ceremony is prescribed, but posts and comrades will in their own way arrange such fitting services and testimonials of respect as circumstances may permit.
 
We are organized, comrades, as our regulations tell us, for the purpose among other things, “of preserving and strengthening those kind and fraternal feelings which have bound together the soldiers, sailors, and marines who united to suppress the late rebellion.” What can aid more to assure this result than cherishing tenderly the memory of our heroic dead, who made their breasts a barricade between our country and its foes? Their soldier lives were the reveille of freedom to a race in chains, and their deaths the tattoo of rebellious tyranny in arms. We should guard their graves with sacred vigilance. All that the consecrated wealth and taste of the nation can add to their adornment and security is but a fitting tribute to the memory of her slain defenders. Let no wanton foot tread rudely on such hallowed grounds. Let pleasant paths invite the coming and going of reverent visitors and fond mourners. Let no vandalism of avarice or neglect, no ravages of time testify to the present or to the coming generations that we have forgotten as a people the cost of a free and undivided republic.
(...)
By order of
JOHN A. LOGAN,
Commander-in-Chief
     
Wikipedia: Memorial Day
Memorial Day is a United States federal holiday which occurs every year on the final Monday of May. Memorial Day is a day of remembering the men and women who died while serving in the United States Armed Forces. Formerly known as Decoration Day, it originated after the American Civil War to commemorate the Union and Confederate soldiers who died in the Civil War.
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Early History
The practice of decorating soldiers’ graves with flowers is an ancient custom. Soldiers’ graves were decorated in the U.S. before and during the American Civil War. A claim was made in 1906 that the first Civil War soldier’s grave ever decorated was in Warrenton, Virginia on June 3, 1861, implying the first Memorial Day occurred there. There is authentic documentation that women in Savannah, Georgia decorated soldiers’ graves in 1862. In 1863, the cemetery dedication at Gettysburg, Pennsylvania was a ceremony of commemoration at the graves of dead soldiers. Local historians in Boalsburg, PA, claim that ladies there decorated soldiers’ graves on July 4, 1864. As a result, Boalsburg promotes itself as the birthplace of Memorial Day.

Following President Abraham Lincoln’s assassination in April 1865, there were a variety of events of commemoration. The first well-known observance of a Memorial Day-type observance after the Civil War was in Charleston, South Carolina on May 1, 1865. During the war, Union soldiers who were prisoners of war had been held at the Charleston Race Course; at least 257 Union prisoners died there and were hastily buried in unmarked graves. Together with teachers and missionaries, black residents of Charleston organized a May Day ceremony in 1865, which was covered by the New York Tribune and other national papers.
 
(Oxford English Dictionary)
Decoration Day (U.S.): the day (now May 30th) kept in memory of those who fell in the civil war of 1861–65, on which their graves are decorated with flowers.
1871 Michigan Gen. Statutes (1882) I. 455   The thirtieth day of May, commonly called decoration day.
1877 Independent 24 May 25/1   Decoration Day dawned bright and beautiful.
 
8 June 1867, Connecticut Courant (Hartford, CT), “Brevities,” pg. 2, col. 5:
On cemetery decoration day in Richmond, the grave of little Joe Davis, son of the ex-, was covered with flowers.
 
26 April 1868, The Daily Picayune (New Orleans, LA), pg. 1, col. 6:
“Decoration Day”—Mississippi Convention.
JACKSON, April 15. To-day was observed by the citizens in the sad but pleasing duty of decorating the graves of the noble dead of the lost cause.
 
26 April 1868, The Daily Constitutionalist (Augusta, GA), “A National Day (From the Mobile Tribune), pg. 1, col. 1:
Let that be our national day.
 
It has been proposed that Mrs. Forrest, wife of the great General, name a day that will be observed as Decoration Day throughout the South. We have not heard whether she has done so or not.
 
The ladies of Louisiana and Mississippi have by tacit agreement adopted the 26th day of April as that on which to perform their sad duties to the dead. On that day of the year 1865 our last hope was buried with the surrender of Johnston’s army.
 
Chronicling America
5 June 1868, The Fremont Weekly Journal (Fremont, OH), pg. 3, col. 3:
Decoration Day Elsewhere.—We have letters from correspondents in Milan, Elmore and Woodville describingthe ceremony indulged at those places last Saturday and Sunday in strewing flowers on the graves of soldiers, which we are unable to find room for.
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Throughout the length and breadth of the country, Decoration Day was observed by the people, and hereafter its annual recurrence will be a national holiday.