Memorial Day (originally known as Decoration Day) is a federal holiday in the United States for mourning the U.S. military personnel who have fought and died while serving in the United States armed forces. It is observed on the last Monday of May; from 1868 to 1970, it was observed on May 30.

Many people visit cemeteries and memorials on Memorial Day to honor and mourn those who fought and died while serving in the U.S. military. Many volunteers place American flags on the graves of military personnel in national cemeteries. Memorial Day is also considered the unofficial beginning of summer in the United States.

The first national observance of Memorial Day occurred on May 30, 1868. Then known as Decoration Day, the holiday was proclaimed by Commander in Chief John A. Logan of the Grand Army of the Republic to honor the Union soldiers who had died in the Civil War. This national observance was preceded by many local ones between the end of the Civil War and Logan's declaration. Many cities and people have claimed to be the first to observe it. However, in 2022, the National Cemetery Administration, a division of the Department of Veterans Affairs, credited Mary Ann Williams with originating the "idea of strewing the graves of Civil War soldiers - Union and Confederate" with flowers.

Official recognition as a holiday spread among the states, beginning with New York in 1873. By 1890, every Union state had adopted it. The World Wars turned it into a day of remembrance for all members of the U.S. military who fought and died in service. In 1971, Congress standardized the holiday as "Memorial Day" and changed its observance to the last Monday in May.

Two other days celebrate those who have served or are serving in the U.S. military: Armed Forces Day (which is earlier in May), an unofficial U.S. holiday for honoring those currently serving in the armed forces, and Veterans Day (on November 11), which honors all those who have served in the United States Armed Forces.

In April 1865, following Lincoln's assassination, commemorations were widespread. The more than 600,000 soldiers of both sides who fought and died in the Civil War meant that burial and memorialization took on new cultural significance. Under women's leadership during the war, the increasingly formal practice of decorating graves had taken shape. In 1865, the federal government began creating the United States National Cemetery System for the Union war dead.

By the 1880s, ceremonies were becoming more consistent across geography as the GAR provided handbooks that presented specific procedures, poems, and Bible verses for local post commanders to utilize in planning the local event. Historian Stuart McConnell reports:

Confederate Memorial Monument in Montgomery, Alabama

In 1868, some Southern public figures added the label "Confederate" to commemorations and claimed Northerners had appropriated the holiday. The first official celebration of Confederate Memorial Day as a public holiday occurred in 1874, following a proclamation by the Georgia legislature. By 1916, ten states celebrated it on June 3, the birthday of CSA President Jefferson Davis. Other states chose late April dates, or May 10, commemorating Davis' capture.

The Ladies Memorial Association played a key role in using Memorial Day rituals to preserve Confederate culture. Various dates ranging from April 25 to mid-June were adopted in different Southern states. Across the South, associations were founded, many by women, to establish and care for permanent cemeteries for the Confederate dead, organize commemorative ceremonies, and sponsor appropriate monuments as a permanent way of remembering the Confederate dead. The most important of these was the United Daughters of the Confederacy, which grew from 17,000 members in 1900 to nearly 100,000 women by World War I. They were "strikingly successful at raising money to build Confederate monuments, lobbying legislatures and Congress for the reburial of Confederate dead, and working to shape the content of history textbooks."

By 1890, there was a shift from the emphasis on honoring specific soldiers to a public commemoration of the Confederate South. Changes in the ceremony's hymns and speeches reflect an evolution of the ritual into a symbol of cultural renewal and conservatism in the South. By 1913, David Blight argues, the theme of American nationalism shared equal time with the Confederate.

By the 20th century, various Union memorial traditions, celebrated on different days, merged, and Memorial Day eventually extended to honor all Americans who fought and died in the U.S. military service. Indiana, from the 1860s to the 1920s, saw numerous debates on how to expand the celebration. It was a favorite lobbying activity of the Grand Army of the Republic (GAR). An 1884 GAR handbook explained that Memorial Day was "the day of all days in the G.A.R. Calendar" in mobilizing public support for pensions. It advised family members to "exercise great care" in keeping the veterans sober.

"On Decoration Day" Political cartoon c. 1900 by John T. McCutcheon. Caption: "You bet I'm goin' to be a soldier, too, like my Uncle David, when I grow up."

Memorial Day speeches became an occasion for veterans, politicians, and ministers to commemorate the Civil War and, at first, to rehash the "atrocities" of the enemy. They mixed religion and celebratory nationalism for the people to make sense of their history in terms of sacrifice for a better nation. People of all religious beliefs joined, and the point was often made that German and Irish soldiers – ethnic minorities who faced discrimination in the United States – had become true Americans in the "baptism of blood" on the battlefield.

In the national capital in 1913, the four-day "Blue-Gray Reunion" featured parades, re-enactments, and speeches from a host of dignitaries, including President Woodrow Wilson, the first Southerner elected to the White House since the War. James Heflin of Alabama gave the main address. Heflin was a noted orator; his choice as Memorial Day speaker was criticized, as he was opposed for his support of segregation; however, his speech was moderate in tone and stressed national unity and goodwill, gaining him praise from newspapers.

The name "Memorial Day," which was first attested in 1882, gradually became more common than "Decoration Day" after World War II but was not declared the official name by federal law until 1967. On June 28, 1968, Congress passed the Uniform Monday Holiday Act, which moved four holidays, including Memorial Day, from traditional dates to a specified Monday to create a convenient three-day weekend. The change moved Memorial Day from its traditional May 30 date to the last Monday in May. The law took effect at the federal level in 1971. After initial confusion and unwillingness to comply, all 50 states adopted Congress's change of date within a few years.

By the early 20th century, the GAR complained more and more about the younger generation. In 1913, one Indiana veteran complained that younger people born since the war had a "tendency ... to forget the purpose of Memorial Day and make it a day for games, races, and revelry, instead of a day of memory and tears". Indeed, in 1911 the scheduling of the Indianapolis Motor Speedway car race (later named the Indianapolis 500) was vehemently opposed by the increasingly elderly GAR. The state legislature in 1923 rejected holding the race on holiday. But the new American Legion and local officials wanted the big race to continue, so Governor Warren McCray vetoed the bill, and the race went on.