"D-Day" is a term that signifies a military operation's starting date, specifically, the Normandy landings during World War II. However, there are several synonymous terms coined to refer to the day or initial stage of an important event or undertaking. Some synonyms include "launch day," "kick-off day," or "beginning day." Others may refer to it as "grand opening," "debut," "commencement," or "starting point." Regardless of its name, the first day of an important mission is often critical and sets the tone for the rest of the endeavor. Thus, it is essential to plan and prepare adequately to ensure a successful outcome.