D-day noun
Learner's definition of D-DAY
[singular]
: a day on which something important is planned or expected to happen重大事件预定发生日；计划行动开始日 ◊ D-day is most commonly used to refer to the specific date of June 6, 1944, when Allied forces began the invasion of France in World War II.第二次世界大战中盟军登陆诺曼底的日子（即1944年6月6日）