Apr 30, 1975, the Fall of Saigon.

Although fighting forces had begun to pull out of Vietnam in 1973, the end of the war didn't come until Apr 30, 1975, when the capitol fell to the PAVN. US forces had been evacuating Vietnamese orphaned children for 3 weeks during Operation Babylift, before the PAVN began its assault on Saigon Apr 29, 1975.

From wiki: "The PAVN, under the command of General Văn Tiến Dũng, began their final attack on Saigon on April 29, 1975, with the Army of the Republic of Vietnam (ARVN) forces commanded by General Nguyễn Văn Toàn suffering a heavy artillery bombardment. By the afternoon of the next day, the PAVN had occupied the important points of the city and raised their flag over the South Vietnamese presidential palace. The city was renamed Hồ Chí Minh City, after the late North Vietnamese President Hồ Chí Minh.

The capture of the city was preceded by Operation Frequent Wind, the evacuation of almost all the American civilian and military personnel in Saigon, along with tens of thousands of South Vietnamese civilians who had been associated with the southern regime. A few Americans chose not to be evacuated. US ground combat units had left South Vietnam more than two years prior to the fall of Saigon and were not available to assist with either the defense of Saigon or the evacuation. The evacuation was the largest helicopter evacuation in history. In addition to the flight of refugees, the end of the war and the institution of new rules by the communists contributed to a decline in the city's population."