The Tet Offensive (Vietnamese: Sự kiện Tết Mậu Thân 1968, or Tổng tiến công và nổi dậy Tết Mậu Thân) was one of the largest military campaigns of the Vietnam War, launched on January 30, 1968, by forces of the Viet Cong and North Vietnamese People’s Army of Vietnam against the forces of the South Vietnamese Army of the Republic of Vietnam, the United States Armed Forces, and their allies.  It was a campaign of surprise attacks against military and civilian command and control centers throughout South Vietnam.  The name of the offensive comes from the Tết holiday, the Vietnamese New Year, when the first major attacks took place.

Source: Wikipedia