the Memorial Day in the US was invented after the civil war, but in most European countries a remembrance/memorial day was introduced around 1919 after WWI.

In Germany there is not only Volkstrauertag, but also Totensonntag, which was introduced 1816 to remember all the people, who died that year. It is unclear whether King Willhelm III of Prussia invented it "because there is no day to remember the dead" after his wife died, or if it was to remember the fallen of the liberation wars. In contrast to Volkstrauertag/Rememberance Day/Memorial Day it is a church holiday

The wonderful PBS documentary Death and the Civil War explains that the availability of spring flowers led both Northerners and Southerners to use the flowers to decorate the graves of their dead in the wake of the War. Southern mourners organized specific 'Decoration Day' events around important dates to the Southern cause, such as the birthday of Jefferson Davis on 3 June.

In 1868, Union General John Logan designated 30 May as the official day to remember the dead of the Union army, and this is the origin of the federal Memorial Day holiday. Some Southern areas continue to this day to observe a separate day for the remembrance of fallen Confederates.

You can watch the relevant part of the documentary here on YouTube.