During World War II, Guam was attacked and invaded by Japan on Monday, December 8, 1941, at the same time as the attack on Pearl Harbor, across the International Date Line. The Japanese renamed Guam (Great Shrine Island). The Japanese occupation of Guam lasted for approximately 31 months. During this period, the indigenous people of Guam were subjected to forced labor, family separation, incarceration, execution, concentration camps and forced prostitution. Approximately 1,000 people died during the occupation, according to later Congressional committee testimony in 2004. Some historians estimate that war violence killed 10% of Guam's then 20,000 population. The United States returned and fought the Battle of Guam from July 21 to August 10, 1944, to recapture the island from Japanese military occupation. July 21 is now celebrated as Liberation Day, a territorial holiday.