Did you know that Memorial Day used to be called Decoration Day? Soldiers' graves were decorated in the U.S. before[and during the American Civil War. Annual Decoration Days for particular cemeteries are still held on a Sunday in late spring or early summer in some rural areas of the American South. In cases involving a family graveyard where remote ancestors as well as those who were deceased more recently are buried, this may take on the character of an extended family reunion to which some people travel hundreds of miles. People gather, put flowers on graves and renew contacts with relatives and others. There often is a religious service and a picnic-like "dinner on the grounds," the traditional term for a potluck meal at a church.
Warrenton, Virginia was the location of the first Civil War soldier's grave ever to be decorated, according to a Richmond Times-Dispatch newspaper article in 1906. In 1862, women in Savannah, Georgia decorated Confederate soldiers' graves according to the Savannah Republican. The 1863 cemetery dedication at Gettysburg, Pennsylvania, was a ceremony of commemoration at the graves of dead soldiers.
In April 1865, following President Abraham Lincoln's assassination, commemorations were ubiquitous. The more than 600,000 soldiers of both sides who died in the Civil War meant that burial and memorialization took on new cultural significance. Under the leadership of women during the war, an increasingly formal practice of decorating graves had taken shape.
In 1868, copying an annual southern observance, General John A. Logan of the Grand Army of the Republic, an organization of Union veterans founded in Decatur, Illinois, established Decoration Day as a time for the nation to decorate the graves of the Union war dead with flowers. By the 20th century, various Union and Confederate memorial traditions, celebrated on different days, merged, and Memorial Day eventually extended to honor all Americans who died while in the military service.
Most people, if they feel so inclined, decorate the graves of veterans in their community graveyards, but if you are a little more ambitious, travel to close closest U.S Military cemetery. You can find a list here: https://www.cem.va.gov/cems/listcem.asp