Memorial Day is actually tomorrow. It originally was called Decoration Day and was a commemoration of the Union AND Confederate soldiers who died in the Civil War. Graves were taken care of, decorated, and families often reunited on the grounds to remember.
The date of Decoration Day was chosen because no particular battle anniversary was on that day. The southern states did not recognize this holiday until after WWI.
As years went by, the holiday was extended to include all military who have died serving our country and thus became Memorial Day. Memorial Day is to not remember the living soldiers but the dead and to be grateful for their ultimate sacrifice.
“The 30th 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 churchyard in the land." ~ General John Logan, Grand Army of the Republic