The Civil War, which ended in the spring of 1865, claimed more lives than any conflict in U.S. history and required the establishment of the country’s first national cemeteries.

By the late 1860s, Americans in various towns and cities had begun holding springtime tributes to these countless fallen soldiers, decorating their graves with flowers and reciting prayers.

It is unclear where exactly this tradition originated; numerous different communities may have independently initiated the memorial gatherings. And some records show that one of the earliest Memorial Day commemorations was organized by a group of formerly enslaved people in Charleston, South Carolina less than a month after the Confederacy surrendered in 1865. Nevertheless, in 1966 the federal government declared Waterloo, New York, the official birthplace of Memorial Day.

Waterloo—which first celebrated the day on May 5, 1866—was chosen because it hosted an annual, community-wide event, during which businesses closed and residents decorated the graves of soldiers with flowers and flags.

On May 5, 1868, General John A. Logan, leader of an organization for Northern Civil War veterans, called for a nationwide day of remembrance later that month. “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,” he proclaimed.

The date of Decoration Day, as he called it, was chosen because it wasn’t the anniversary of any particular battle.

On the first Decoration Day, General James Garfield made a speech at Arlington National Cemetery, and 5,000 participants decorated the graves of the 20,000 Civil War soldiers buried there.

Many Northern states held similar commemorative events and reprised the tradition in subsequent years; by 1890 each one had made Decoration Day an official state holiday. Southern states, on the other hand, continued to honor the dead on separate days until after World War I.

Memorial Day, as Decoration Day gradually came to be known, originally honored only those lost while fighting in the Civil War. But during World War I the United States found itself embroiled in another major conflict, and the holiday evolved to commemorate American military personnel who died in all wars, including World War II, The Vietnam War, The Korean War and the wars in Iraq and Afghanistan.

For decades, Memorial Day continued to be observed on May 30, the date General Logan had selected for the first Decoration Day. But in 1968, Congress passed the Uniform Monday Holiday Act, which established Memorial Day as the last Monday in May in order to create a three-day weekend for federal employees. The change went into effect in 1971. The same law also declared Memorial Day a federal holiday.

Cities and towns across the United States host Memorial Day parades each year, often incorporating military personnel and members of veterans’ organizations. Some of the largest parades take place in Chicago, New York and Washington, D.C.

Americans also observe Memorial Day by visiting cemeteries and memorials. Some people wear a red poppy in remembrance of those fallen in war—a tradition that began with a World War I poem. On a less somber note, many people take weekend trips or throw parties and barbecues on the holiday, perhaps because Memorial Day weekend—the long weekend comprising the Saturday and Sunday before Memorial Day and Memorial Day itself—unofficially marks the beginning of summer.