Memorial Day

Fold3 has an article about 30 May 1868, the first official observance of Memorial Day:

In the years immediately following the Civil War, one way Americans sought to remember the multitudes of war dead was by holding decoration days, on which they would gather to decorate the graves of those who died in the conflict. Although many local groups and communities had their own decoration days, including well-known ones in Waterloo, New York) and Charleston, South Carolina), the first official observance of what would eventually become Memorial Day took place on 30 May 1868.
Decoration Day (it wouldn't officially be called Memorial Day until 1967) was coordinated by John A. Logan, a former Union general and, at the time, commander-in-chief of the Grand Army of the Republic (GAR), a Union veterans' association. In his General Order Number 11, dated 5 May 1868, he designated 30 May 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. In addition to the decoration of graves, Decoration Day was also to be observed with 'fitting services and testimonials of respect as circumstances may permit', according to Logan.
Veterans and their loved ones, as well as widows, orphans, and other bereaved, responded to Logan's call with alacrity. That year, 183 cemeteries in 27 states celebrated Decoration Day, and observance grew in the years that followed. By 1890, all the northern states had made it an official state holiday.
The South didn't celebrate Logan's Decoration Day until after World War One, when the holiday shifted from honoring Civil War dead to honoring the American dead of all wars. Instead, Southerners memorialized the Confederate dead locally on days throughout the spring and early summer, often on important dates such as Joseph Johnston's surrender, Stonewall Jackson's death, or Jefferson Davis' birthday.
Memorial Day, in the form we know it today, came about in 1967, when Decoration Day was renamed Memorial Day, to better reflect contemporary usage. The following year it was permanently moved from 30 May to the last Monday in May to create a three-day weekend.