You can use D-day to refer to the day that is chosen for the beginning of an important activity.
D-day for my departure was set for 29th June.
D-day in British English
noun
1.
the day, June 6, 1944, in World War II on which the Allies invaded France in a series of beach landings in Normandy
2.
the day on which any large-scale operation is planned to start
Word origin
C20: from D(ay)-day; compare H-hour
D-day in American English
(ˈdiˌdeɪ) or D-Day (ˈdiˌdeɪ)
noun
the day on which a military attack or other important event is to take place; specif., June 6, 1944, the day of theinvasion of W Europe by Allied forces in WWII
Word origin
D, the first letter of day
Examples of 'D-day' in a sentence
D-day
Maybe in the last few months before D-day I'll start trying to be good.
Robert Wilson A DARKENING STAIN (2002)
She had said the same thing the year before when he'd gone to Europe without her for the fortieth anniversary of D-day.