Originally called Decoration Day, this holiday is a day of remembrance for those who have died in service of the United States of America. Memorial Day was first observed after the Civil War as a desire to honor the dead. Originally proclaimed on 5 May 1868 by General John Logan (national commander of the Grand Army of the Republic in the Civil War) because the date wasn't the anniversary of any particular battle. In his General Order No. 11 he proclaimed:
“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,” (original text
link here)