start day for military operation; departure day; D-Day
Military term for the scheduled day a military operation begins; can also be used metaphorically for any important launch day.
The start day for the operation has been set for next Monday.
We announced the project's launch day internally.
Dデー is a direct loan from English 'D-Day' and often refers specifically to the Normandy landings or a decisive day, while 作戦開始日 is a more general Japanese term for any operation start date.
Compound of 作戦 (sakusen, 'operation') + 開始 (kaishi, 'start') + 日 (hi, 'day'). The reading ひ becomes び due to rendaku.