D-Day
noun [ C usually singular ] uk/ˈdiː.deɪ/ us/ˈdiː.deɪ/
（二战期间同盟国进攻欧洲的）登陆日
the day during the Second World War when the Allies began their invasion of Europe by attacking the coast of northern France
The D-Day landings began on 6 June 1944, when Allied forces invaded Normandy. 登陆日的登陆行动是从1944年6月6日同盟国军队进攻诺曼底开始的。
大日子，重要的日子
a day when something important will happen
After four hectic weeks of electioneering, candidates are preparing themselves for D-Day (= election day) tomorrow. 经历了4周忙碌的竞选活动后，候选人正在为明天的选举投票作准备。