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。

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。