Answer
The answer is two days. Dương Văn Minh served as the President of South Vietnam for only two days in April 1975, during the chaotic final days of the Vietnam War. He assumed the presidency on April 21st, after President Nguyễn Văn Thiệu resigned and fled the country. Minh immediately called for a ceasefire and unconditional surrender to North Vietnam, effectively ending the war.