Definitions:

1. A day set to launch an operation. NOUN
   • The Allied forces launched the largest seaborne invasion in history on D-Day, June 6, 1944, marking the beginning of the end of World War II in Europe.
2. An Army designation used to indicate the start date for specific field operations. NOUN
   • The strategic planning and execution of D-Day demonstrated remarkable coordination among the Allied nations.
3. The day in World War II on which Allied forces invaded northern France by means of beach landings in Normandy. NOUN
   • Veterans of D-Day often share harrowing and heroic stories of their experiences during the Normandy landings.
4. The day on which an important operation is to begin or a change to take place. NOUN
   • It's D-day at the Websters', as Sally gives Kevin an ultimatum.

Synonyms:

• h-hour
• invasion day
• normandy landings
• operation overlord
• the longest day