The code name for the first day of a military attack. Fig.: D-day refers to the day chosen for the beginning of an important activity or for an important change

I finally made up my mind: I'm giving up smoking and tomorrow will be D-day

Finalmente ho deciso: smetto di fumare e domani sarà il giorno X

So you are getting married! When is the D-day?

E così ti sposi! Quando è il giorno fatidico?

Specifically, the code name applies to the day of June 6, 1944, when, during World War II, American and British forces invaded German-occupied northern France by means of beach landing in Normandy. The invasion marked the beginning of the Allied victory in Europe; Germany surrendered less than a year later