Liberation Day on the U.S. territory of Guam is an annual commemoration of the invasion by U.S. military forces on July 21, 1944, which ended the Japanese occupation that had begun in 1941. Begun in 1945, it is Guam's largest celebration. Festivities include a queen contest, summer carnival, fireworks display, and mile-long parade on Marine Corps Drive in Hagåtña from Adelup to Paseo de Susana, as well as solemn memorials and visits to massacre sites. It is organized by the Guam Island Fair Committee.

Before the Japanese capture of Guam on December 10, 1941, the Guam Island Fair was celebrated annually, first in the Plaza de España in Hagåtña and later in Bradley Park in Anigua. With tensions rising after the 1931 Japanese invasion of Manchuria starting the Second Sino-Japanese War, a committee formed in 1938 by U.S. Rear Admiral Arthur Japy Hepburn recommended a massive buildup of Guam as an air and submarine base. However, the U.S. balked at the estimated cost. American military dependents and contractors were evacuated weeks before the invasion. While Guamanians were nationals of the United States, they would not become citizens until the passage of the Guam Organic Act of 1950.^{[citation needed]} Practically defenseless, U.S. and territorial forces made token resistance before surrendering to the overwhelming Japanese invasion force, becoming one of five parts of the U.S. captured by the Japanese in World War II, along with the Philippines, Wake Island and the Aleutian Islands of Kiska and Attu.

Renamed Omiya Jima, Japanese for "Great Shrine Island," the island was administered as part of the Greater East Asia Co-Prosperity Sphere by the Imperial Japanese Navy, with the ultimate intent to make it part of the Saipan Branch of the South Seas Bureau. In the early part of the occupation, Japanese forces harassed and tortured Chamorro residents who they suspected of aiding the few American military personnel hiding on Guam, but largely focused on administration aimed at incorporating the island, such as teaching Japanese language in schools. This changed as the Pacific War drew closer and the threat of American attack increased. In early 1944, between 10,000 and 15,000 Chamorros were forced to march with only the belongings they could carry to internment camps in the middle of the island. Most of the male population was forced to build infrastructure and defenses, including the two airfields that became Orote Field and Naval Air Station Agana, which is now Antonio B. Won Pat International Airport. In 1944, the Japanese response to American aircraft's preparatory bombing from June 11 to 13, followed by naval bombardment starting on June 27, was brutal. Japanese massacred dozens of Chamorro civilians in Fena, Merizo, and Yigo. As the invasion neared, Japanese soldiers indiscriminately killed any Chamorros they encountered around the island. The Japanese also raped numerous women around the island. Meanwhile, an unknown number were killed by the American aerial and naval bombardment.

On July 21, 1944 the 3rd Marine Division landed at Asan and the 1st Provisional Marine Brigade landed at Agat to the south, on either side of the strategically important Apra Harbor and Orote Peninsula. Together with the U.S. Army's 77th Infantry Division, and the support of U.S. Navy Seabees, American forces captured the Orote Peninsula by July 29 and pushed north. On August 10, organized Japanese resistance ended, though the last Japanese holdout, Shoichi Yokoi, was not captured until 1972.

In 1945 - the immediate aftermath of the war - prominent Chamorro educator Agueda Iglesias Johnston was instrumental in convincing the island's leadership to support a commemorative celebration of the island's liberation. Johnston had influence with the Army and Navy commands, but had a harder time convincing civilian leaders that a celebration was needed. The military supplied a 100-pound cake, a band, and transportation but Johnston was responsible for a venue. She found an old school with walls and floors made of bamboo and recalled, "When the young people began the jitterbug, I feared some of them would be hurled through the walls. No one was hurt."

The first Liberation Queen was named in 1948. As with the pre-war Guam Island Fair, the Queen is named based on ticket sales, with the runner-up named Liberation Princess.

The Liberation Queen contest, parade, carnival, and religious and memorial services have been constant features of an often multi-day event. However, other kinds of celebrations have been done over the years. In 1950, the Liberation Day program called for a morning Mass, the decoration of graves and war markers, and then an informal dance. In 1952, three days of festivities, with fireworks nightly, included a motorboat parade, horse racing, carabao racing, athletic competitions, and a boxing tournament. Liberation Day festivities have including wrestling matches, agricultural fairs, bicycle races, and pig wrestling, with instances of a Battle of the Bands in 1971 and a circus. The parade route begins at Adelup and typically runs to Paseo de Susana, but in 1976 it went to Camp Asan and in 1983 it went to Hagåtña Pool. While the carnival is typically at Paseo, it has been held at the former Yigo Amusement Park, the Harmon Cliff line, and Tiyan in the past. In 1995, PBS Guam began a yearly tradition of live broadcasts of the parade.

For the milestone 25th anniversary in 1969, a special six-day celebration was held. A Marine Corps band from Honolulu was flown in and a 30-piece Army band performed. The grand marshal was Marine Corps Brig. Gen. Louis H. Wilson Jr., who had been awarded the Medal of Honor for his actions during the Battle of Guam and would later become the Commandant of the Marine Corps.