Originally, Memorial Day was called Decoration Day, and it honored only the Civil War dead. The Teaching with the Library of Congress blog says:
In 1868, John Logan, the Commander in Chief of the Grand Army of the Republic, an organization for Union veterans, issued an order designating May 30th as a memorial day. He said this day should be 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 churchyard in the land.” Logan also asked that we guard their graves so that future generations can remember the cost of a free and undivided republic.
The Library of Congress has a treasure trove of primary sources online that tie into Memorial Day. Students can use these to learn how Memorial Day has been observed in the past. Learn more from the Library of Congress.